Move `html` module to `typst-html` crate (#6577)

4 files changed, 877 insertions, 2 deletions

diff --git a/crates/typst-html/Cargo.toml b/crates/typst-html/Cargo.toml
index 534848f9..54cad012 100644
--- a/crates/typst-html/Cargo.toml
+++ b/crates/typst-html/Cargo.toml
@@ -13,14 +13,18 @@ keywords = { workspace = true }
 readme = { workspace = true }
 
 [dependencies]
+typst-assets = { workspace = true }
 typst-library = { workspace = true }
 typst-macros = { workspace = true }
 typst-syntax = { workspace = true }
 typst-timing = { workspace = true }
 typst-utils = { workspace = true }
 typst-svg = { workspace = true }
+bumpalo = { workspace = true }
 comemo = { workspace = true }
 ecow = { workspace = true }
+palette = { workspace = true }
+time = { workspace = true }
 
 [lints]
 workspace = true
diff --git a/crates/typst-html/src/css.rs b/crates/typst-html/src/css.rs
new file mode 100644
index 00000000..2b659188
--- /dev/null
+++ b/crates/typst-html/src/css.rs
@@ -0,0 +1,135 @@
+//! Conversion from Typst data types into CSS data types.
+
+use std::fmt::{self, Display};
+
+use typst_library::layout::Length;
+use typst_library::visualize::{Color, Hsl, LinearRgb, Oklab, Oklch, Rgb};
+use typst_utils::Numeric;
+
+pub fn length(length: Length) -> impl Display {
+    typst_utils::display(move |f| match (length.abs.is_zero(), length.em.is_zero()) {
+        (false, false) => {
+            write!(f, "calc({}pt + {}em)", length.abs.to_pt(), length.em.get())
+        }
+        (true, false) => write!(f, "{}em", length.em.get()),
+        (_, true) => write!(f, "{}pt", length.abs.to_pt()),
+    })
+}
+
+pub fn color(color: Color) -> impl Display {
+    typst_utils::display(move |f| match color {
+        Color::Rgb(_) | Color::Cmyk(_) | Color::Luma(_) => rgb(f, color.to_rgb()),
+        Color::Oklab(v) => oklab(f, v),
+        Color::Oklch(v) => oklch(f, v),
+        Color::LinearRgb(v) => linear_rgb(f, v),
+        Color::Hsl(_) | Color::Hsv(_) => hsl(f, color.to_hsl()),
+    })
+}
+
+fn oklab(f: &mut fmt::Formatter<'_>, v: Oklab) -> fmt::Result {
+    write!(f, "oklab({} {} {}{})", percent(v.l), number(v.a), number(v.b), alpha(v.alpha))
+}
+
+fn oklch(f: &mut fmt::Formatter<'_>, v: Oklch) -> fmt::Result {
+    write!(
+        f,
+        "oklch({} {} {}deg{})",
+        percent(v.l),
+        number(v.chroma),
+        number(v.hue.into_degrees()),
+        alpha(v.alpha)
+    )
+}
+
+fn rgb(f: &mut fmt::Formatter<'_>, v: Rgb) -> fmt::Result {
+    if let Some(v) = rgb_to_8_bit_lossless(v) {
+        let (r, g, b, a) = v.into_components();
+        write!(f, "#{r:02x}{g:02x}{b:02x}")?;
+        if a != u8::MAX {
+            write!(f, "{a:02x}")?;
+        }
+        Ok(())
+    } else {
+        write!(
+            f,
+            "rgb({} {} {}{})",
+            percent(v.red),
+            percent(v.green),
+            percent(v.blue),
+            alpha(v.alpha)
+        )
+    }
+}
+
+/// Converts an f32 RGBA color to its 8-bit representation if the result is
+/// [very close](is_very_close) to the original.
+fn rgb_to_8_bit_lossless(
+    v: Rgb,
+) -> Option<palette::rgb::Rgba<palette::encoding::Srgb, u8>> {
+    let l = v.into_format::<u8, u8>();
+    let h = l.into_format::<f32, f32>();
+    (is_very_close(v.red, h.red)
+        && is_very_close(v.blue, h.blue)
+        && is_very_close(v.green, h.green)
+        && is_very_close(v.alpha, h.alpha))
+    .then_some(l)
+}
+
+fn linear_rgb(f: &mut fmt::Formatter<'_>, v: LinearRgb) -> fmt::Result {
+    write!(
+        f,
+        "color(srgb-linear {} {} {}{})",
+        percent(v.red),
+        percent(v.green),
+        percent(v.blue),
+        alpha(v.alpha),
+    )
+}
+
+fn hsl(f: &mut fmt::Formatter<'_>, v: Hsl) -> fmt::Result {
+    write!(
+        f,
+        "hsl({}deg {} {}{})",
+        number(v.hue.into_degrees()),
+        percent(v.saturation),
+        percent(v.lightness),
+        alpha(v.alpha),
+    )
+}
+
+/// Displays an alpha component if it not 1.
+fn alpha(value: f32) -> impl Display {
+    typst_utils::display(move |f| {
+        if !is_very_close(value, 1.0) {
+            write!(f, " / {}", percent(value))?;
+        }
+        Ok(())
+    })
+}
+
+/// Displays a rounded percentage.
+///
+/// For a percentage, two significant digits after the comma gives us a
+/// precision of 1/10_000, which is more than 12 bits (see `is_very_close`).
+fn percent(ratio: f32) -> impl Display {
+    typst_utils::display(move |f| {
+        write!(f, "{}%", typst_utils::round_with_precision(ratio as f64 * 100.0, 2))
+    })
+}
+
+/// Rounds a number for display.
+///
+/// For a number between 0 and 1, four significant digits give us a
+/// precision of 1/10_000, which is more than 12 bits (see `is_very_close`).
+fn number(value: f32) -> impl Display {
+    typst_utils::round_with_precision(value as f64, 4)
+}
+
+/// Whether two component values are close enough that there is no
+/// difference when encoding them with 12-bit. 12 bit is the highest
+/// reasonable color bit depth found in the industry.
+fn is_very_close(a: f32, b: f32) -> bool {
+    const MAX_BIT_DEPTH: u32 = 12;
+    const EPS: f32 = 0.5 / 2_i32.pow(MAX_BIT_DEPTH) as f32;
+    (a - b).abs() < EPS
+}
diff --git a/crates/typst-html/src/lib.rs b/crates/typst-html/src/lib.rs
index 19eb1446..7063931b 100644
--- a/crates/typst-html/src/lib.rs
+++ b/crates/typst-html/src/lib.rs
@@ -1,7 +1,9 @@
 //! Typst's HTML exporter.
 
+mod css;
 mod encode;
 mod rules;
+mod typed;
 
 pub use self::encode::html;
 pub use self::rules::register;
@@ -9,7 +11,9 @@ pub use self::rules::register;
 use comemo::{Track, Tracked, TrackedMut};
 use typst_library::diag::{bail, warning, At, SourceResult};
 use typst_library::engine::{Engine, Route, Sink, Traced};
-use typst_library::foundations::{Content, StyleChain, Target, TargetElem};
+use typst_library::foundations::{
+    Content, Module, Scope, StyleChain, Target, TargetElem,
+};
 use typst_library::html::{
     attr, tag, FrameElem, HtmlDocument, HtmlElem, HtmlElement, HtmlFrame, HtmlNode,
 };
@@ -20,9 +24,19 @@ use typst_library::layout::{Abs, Axes, BlockBody, BlockElem, BoxElem, Region, Si
 use typst_library::model::{DocumentInfo, ParElem};
 use typst_library::routines::{Arenas, FragmentKind, Pair, RealizationKind, Routines};
 use typst_library::text::{LinebreakElem, SmartQuoteElem, SpaceElem, TextElem};
-use typst_library::World;
+use typst_library::{Category, World};
 use typst_syntax::Span;
 
+/// Create a module with all HTML definitions.
+pub fn module() -> Module {
+    let mut html = Scope::deduplicating();
+    html.start_category(Category::Html);
+    html.define_elem::<HtmlElem>();
+    html.define_elem::<FrameElem>();
+    crate::typed::define(&mut html);
+    Module::new("html", html)
+}
+
 /// Produce an HTML document from content.
 ///
 /// This first performs root-level realization and then turns the resulting
diff --git a/crates/typst-html/src/typed.rs b/crates/typst-html/src/typed.rs
new file mode 100644
index 00000000..4b794bbb
--- /dev/null
+++ b/crates/typst-html/src/typed.rs
@@ -0,0 +1,722 @@
+//! The typed HTML element API (e.g. `html.div`).
+//!
+//! The typed API is backed by generated data derived from the HTML
+//! specification. See [generated] and `tools/codegen`.
+
+use std::fmt::Write;
+use std::num::{NonZeroI64, NonZeroU64};
+use std::sync::LazyLock;
+
+use bumpalo::Bump;
+use comemo::Tracked;
+use ecow::{eco_format, eco_vec, EcoString};
+use typst_assets::html as data;
+use typst_library::diag::{bail, At, Hint, HintedStrResult, SourceResult};
+use typst_library::engine::Engine;
+use typst_library::foundations::{
+    Args, Array, AutoValue, CastInfo, Content, Context, Datetime, Dict, Duration,
+    FromValue, IntoValue, NativeFuncData, NativeFuncPtr, NoneValue, ParamInfo,
+    PositiveF64, Reflect, Scope, Str, Type, Value,
+};
+use typst_library::html::tag;
+use typst_library::html::{HtmlAttr, HtmlAttrs, HtmlElem, HtmlTag};
+use typst_library::layout::{Axes, Axis, Dir, Length};
+use typst_library::visualize::Color;
+use typst_macros::cast;
+
+use crate::css;
+
+/// Hook up all typed HTML definitions.
+pub(super) fn define(html: &mut Scope) {
+    for data in FUNCS.iter() {
+        html.define_func_with_data(data);
+    }
+}
+
+/// Lazily created functions for all typed HTML constructors.
+static FUNCS: LazyLock<Vec<NativeFuncData>> = LazyLock::new(|| {
+    // Leaking is okay here. It's not meaningfully different from having
+    // memory-managed values as `FUNCS` is a static.
+    let bump = Box::leak(Box::new(Bump::new()));
+    data::ELEMS.iter().map(|info| create_func_data(info, bump)).collect()
+});
+
+/// Creates metadata for a native HTML element constructor function.
+fn create_func_data(
+    element: &'static data::ElemInfo,
+    bump: &'static Bump,
+) -> NativeFuncData {
+    NativeFuncData {
+        function: NativeFuncPtr(bump.alloc(
+            move |_: &mut Engine, _: Tracked<Context>, args: &mut Args| {
+                construct(element, args)
+            },
+        )),
+        name: element.name,
+        title: {
+            let title = bump.alloc_str(element.name);
+            title[0..1].make_ascii_uppercase();
+            title
+        },
+        docs: element.docs,
+        keywords: &[],
+        contextual: false,
+        scope: LazyLock::new(&|| Scope::new()),
+        params: LazyLock::new(bump.alloc(move || create_param_info(element))),
+        returns: LazyLock::new(&|| CastInfo::Type(Type::of::<Content>())),
+    }
+}
+
+/// Creates parameter signature metadata for an element.
+fn create_param_info(element: &'static data::ElemInfo) -> Vec<ParamInfo> {
+    let mut params = vec![];
+    for attr in element.attributes() {
+        params.push(ParamInfo {
+            name: attr.name,
+            docs: attr.docs,
+            input: AttrType::convert(attr.ty).input(),
+            default: None,
+            positional: false,
+            named: true,
+            variadic: false,
+            required: false,
+            settable: false,
+        });
+    }
+    let tag = HtmlTag::constant(element.name);
+    if !tag::is_void(tag) {
+        params.push(ParamInfo {
+            name: "body",
+            docs: "The contents of the HTML element.",
+            input: CastInfo::Type(Type::of::<Content>()),
+            default: None,
+            positional: true,
+            named: false,
+            variadic: false,
+            required: false,
+            settable: false,
+        });
+    }
+    params
+}
+
+/// The native constructor function shared by all HTML elements.
+fn construct(element: &'static data::ElemInfo, args: &mut Args) -> SourceResult<Value> {
+    let mut attrs = HtmlAttrs::default();
+    let mut errors = eco_vec![];
+
+    args.items.retain(|item| {
+        let Some(name) = &item.name else { return true };
+        let Some(attr) = element.get_attr(name) else { return true };
+
+        let span = item.value.span;
+        let value = std::mem::take(&mut item.value.v);
+        let ty = AttrType::convert(attr.ty);
+        match ty.cast(value).at(span) {
+            Ok(Some(string)) => attrs.push(HtmlAttr::constant(attr.name), string),
+            Ok(None) => {}
+            Err(diags) => errors.extend(diags),
+        }
+
+        false
+    });
+
+    if !errors.is_empty() {
+        return Err(errors);
+    }
+
+    let tag = HtmlTag::constant(element.name);
+    let mut elem = HtmlElem::new(tag);
+    if !attrs.0.is_empty() {
+        elem.attrs.set(attrs);
+    }
+
+    if !tag::is_void(tag) {
+        let body = args.eat::<Content>()?;
+        elem.body.set(body);
+    }
+
+    Ok(elem.into_value())
+}
+
+/// A dynamic representation of an attribute's type.
+///
+/// See the documentation of [`data::Type`] for more details on variants.
+enum AttrType {
+    Presence,
+    Native(NativeType),
+    Strings(StringsType),
+    Union(UnionType),
+    List(ListType),
+}
+
+impl AttrType {
+    /// Converts the type definition into a representation suitable for casting
+    /// and reflection.
+    const fn convert(ty: data::Type) -> AttrType {
+        use data::Type;
+        match ty {
+            Type::Presence => Self::Presence,
+            Type::None => Self::of::<NoneValue>(),
+            Type::NoneEmpty => Self::of::<NoneEmpty>(),
+            Type::NoneUndefined => Self::of::<NoneUndefined>(),
+            Type::Auto => Self::of::<AutoValue>(),
+            Type::TrueFalse => Self::of::<TrueFalseBool>(),
+            Type::YesNo => Self::of::<YesNoBool>(),
+            Type::OnOff => Self::of::<OnOffBool>(),
+            Type::Int => Self::of::<i64>(),
+            Type::NonNegativeInt => Self::of::<u64>(),
+            Type::PositiveInt => Self::of::<NonZeroU64>(),
+            Type::Float => Self::of::<f64>(),
+            Type::PositiveFloat => Self::of::<PositiveF64>(),
+            Type::Str => Self::of::<Str>(),
+            Type::Char => Self::of::<char>(),
+            Type::Datetime => Self::of::<Datetime>(),
+            Type::Duration => Self::of::<Duration>(),
+            Type::Color => Self::of::<Color>(),
+            Type::HorizontalDir => Self::of::<HorizontalDir>(),
+            Type::IconSize => Self::of::<IconSize>(),
+            Type::ImageCandidate => Self::of::<ImageCandidate>(),
+            Type::SourceSize => Self::of::<SourceSize>(),
+            Type::Strings(start, end) => Self::Strings(StringsType { start, end }),
+            Type::Union(variants) => Self::Union(UnionType(variants)),
+            Type::List(inner, separator, shorthand) => {
+                Self::List(ListType { inner, separator, shorthand })
+            }
+        }
+    }
+
+    /// Produces the dynamic representation of an attribute type backed by a
+    /// native Rust type.
+    const fn of<T: IntoAttr>() -> Self {
+        Self::Native(NativeType::of::<T>())
+    }
+
+    /// See [`Reflect::input`].
+    fn input(&self) -> CastInfo {
+        match self {
+            Self::Presence => bool::input(),
+            Self::Native(ty) => (ty.input)(),
+            Self::Union(ty) => ty.input(),
+            Self::Strings(ty) => ty.input(),
+            Self::List(ty) => ty.input(),
+        }
+    }
+
+    /// See [`Reflect::castable`].
+    fn castable(&self, value: &Value) -> bool {
+        match self {
+            Self::Presence => bool::castable(value),
+            Self::Native(ty) => (ty.castable)(value),
+            Self::Union(ty) => ty.castable(value),
+            Self::Strings(ty) => ty.castable(value),
+            Self::List(ty) => ty.castable(value),
+        }
+    }
+
+    /// Tries to cast the value into this attribute's type and serialize it into
+    /// an HTML attribute string.
+    fn cast(&self, value: Value) -> HintedStrResult<Option<EcoString>> {
+        match self {
+            Self::Presence => value.cast::<bool>().map(|b| b.then(EcoString::new)),
+            Self::Native(ty) => (ty.cast)(value),
+            Self::Union(ty) => ty.cast(value),
+            Self::Strings(ty) => ty.cast(value),
+            Self::List(ty) => ty.cast(value),
+        }
+    }
+}
+
+/// An enumeration with generated string variants.
+///
+/// `start` and `end` are used to index into `data::ATTR_STRINGS`.
+struct StringsType {
+    start: usize,
+    end: usize,
+}
+
+impl StringsType {
+    fn input(&self) -> CastInfo {
+        CastInfo::Union(
+            self.strings()
+                .iter()
+                .map(|(val, desc)| CastInfo::Value(val.into_value(), desc))
+                .collect(),
+        )
+    }
+
+    fn castable(&self, value: &Value) -> bool {
+        match value {
+            Value::Str(s) => self.strings().iter().any(|&(v, _)| v == s.as_str()),
+            _ => false,
+        }
+    }
+
+    fn cast(&self, value: Value) -> HintedStrResult<Option<EcoString>> {
+        if self.castable(&value) {
+            value.cast().map(Some)
+        } else {
+            Err(self.input().error(&value))
+        }
+    }
+
+    fn strings(&self) -> &'static [(&'static str, &'static str)] {
+        &data::ATTR_STRINGS[self.start..self.end]
+    }
+}
+
+/// A type that accepts any of the contained types.
+struct UnionType(&'static [data::Type]);
+
+impl UnionType {
+    fn input(&self) -> CastInfo {
+        CastInfo::Union(self.iter().map(|ty| ty.input()).collect())
+    }
+
+    fn castable(&self, value: &Value) -> bool {
+        self.iter().any(|ty| ty.castable(value))
+    }
+
+    fn cast(&self, value: Value) -> HintedStrResult<Option<EcoString>> {
+        for item in self.iter() {
+            if item.castable(&value) {
+                return item.cast(value);
+            }
+        }
+        Err(self.input().error(&value))
+    }
+
+    fn iter(&self) -> impl Iterator<Item = AttrType> {
+        self.0.iter().map(|&ty| AttrType::convert(ty))
+    }
+}
+
+/// A list of items separated by a specific separator char.
+///
+/// - <https://html.spec.whatwg.org/#space-separated-tokens>
+/// - <https://html.spec.whatwg.org/#comma-separated-tokens>
+struct ListType {
+    inner: &'static data::Type,
+    separator: char,
+    shorthand: bool,
+}
+
+impl ListType {
+    fn input(&self) -> CastInfo {
+        if self.shorthand {
+            Array::input() + self.inner().input()
+        } else {
+            Array::input()
+        }
+    }
+
+    fn castable(&self, value: &Value) -> bool {
+        Array::castable(value) || (self.shorthand && self.inner().castable(value))
+    }
+
+    fn cast(&self, value: Value) -> HintedStrResult<Option<EcoString>> {
+        let ty = self.inner();
+        if Array::castable(&value) {
+            let array = value.cast::<Array>()?;
+            let mut out = EcoString::new();
+            for (i, item) in array.into_iter().enumerate() {
+                let item = ty.cast(item)?.unwrap();
+                if item.as_str().contains(self.separator) {
+                    let buf;
+                    let name = match self.separator {
+                        ' ' => "space",
+                        ',' => "comma",
+                        _ => {
+                            buf = eco_format!("'{}'", self.separator);
+                            buf.as_str()
+                        }
+                    };
+                    bail!(
+                        "array item may not contain a {name}";
+                        hint: "the array attribute will be encoded as a \
+                               {name}-separated string"
+                    );
+                }
+                if i > 0 {
+                    out.push(self.separator);
+                    if self.separator == ',' {
+                        out.push(' ');
+                    }
+                }
+                out.push_str(&item);
+            }
+            Ok(Some(out))
+        } else if self.shorthand && ty.castable(&value) {
+            let item = ty.cast(value)?.unwrap();
+            Ok(Some(item))
+        } else {
+            Err(self.input().error(&value))
+        }
+    }
+
+    fn inner(&self) -> AttrType {
+        AttrType::convert(*self.inner)
+    }
+}
+
+/// A dynamic representation of attribute backed by a native type implementing
+/// - the standard `Reflect` and `FromValue` traits for casting from a value,
+/// - the special `IntoAttr` trait for conversion into an attribute string.
+#[derive(Copy, Clone)]
+struct NativeType {
+    input: fn() -> CastInfo,
+    cast: fn(Value) -> HintedStrResult<Option<EcoString>>,
+    castable: fn(&Value) -> bool,
+}
+
+impl NativeType {
+    /// Creates a dynamic native type from a native Rust type.
+    const fn of<T: IntoAttr>() -> Self {
+        Self {
+            cast: |value| {
+                let this = value.cast::<T>()?;
+                Ok(Some(this.into_attr()))
+            },
+            input: T::input,
+            castable: T::castable,
+        }
+    }
+}
+
+/// Casts a native type into an HTML attribute.
+pub trait IntoAttr: FromValue {
+    /// Turn the value into an attribute string.
+    fn into_attr(self) -> EcoString;
+}
+
+impl IntoAttr for Str {
+    fn into_attr(self) -> EcoString {
+        self.into()
+    }
+}
+
+/// A boolean that is encoded as a string:
+/// - `false` is encoded as `"false"`
+/// - `true` is encoded as `"true"`
+pub struct TrueFalseBool(pub bool);
+
+cast! {
+    TrueFalseBool,
+    v: bool => Self(v),
+}
+
+impl IntoAttr for TrueFalseBool {
+    fn into_attr(self) -> EcoString {
+        if self.0 { "true" } else { "false" }.into()
+    }
+}
+
+/// A boolean that is encoded as a string:
+/// - `false` is encoded as `"no"`
+/// - `true` is encoded as `"yes"`
+pub struct YesNoBool(pub bool);
+
+cast! {
+    YesNoBool,
+    v: bool => Self(v),
+}
+
+impl IntoAttr for YesNoBool {
+    fn into_attr(self) -> EcoString {
+        if self.0 { "yes" } else { "no" }.into()
+    }
+}
+
+/// A boolean that is encoded as a string:
+/// - `false` is encoded as `"off"`
+/// - `true` is encoded as `"on"`
+pub struct OnOffBool(pub bool);
+
+cast! {
+    OnOffBool,
+    v: bool => Self(v),
+}
+
+impl IntoAttr for OnOffBool {
+    fn into_attr(self) -> EcoString {
+        if self.0 { "on" } else { "off" }.into()
+    }
+}
+
+impl IntoAttr for AutoValue {
+    fn into_attr(self) -> EcoString {
+        "auto".into()
+    }
+}
+
+impl IntoAttr for NoneValue {
+    fn into_attr(self) -> EcoString {
+        "none".into()
+    }
+}
+
+/// A `none` value that turns into an empty string attribute.
+struct NoneEmpty;
+
+cast! {
+    NoneEmpty,
+    _: NoneValue => NoneEmpty,
+}
+
+impl IntoAttr for NoneEmpty {
+    fn into_attr(self) -> EcoString {
+        "".into()
+    }
+}
+
+/// A `none` value that turns into the string `"undefined"`.
+struct NoneUndefined;
+
+cast! {
+    NoneUndefined,
+    _: NoneValue => NoneUndefined,
+}
+
+impl IntoAttr for NoneUndefined {
+    fn into_attr(self) -> EcoString {
+        "undefined".into()
+    }
+}
+
+impl IntoAttr for char {
+    fn into_attr(self) -> EcoString {
+        eco_format!("{self}")
+    }
+}
+
+impl IntoAttr for i64 {
+    fn into_attr(self) -> EcoString {
+        eco_format!("{self}")
+    }
+}
+
+impl IntoAttr for u64 {
+    fn into_attr(self) -> EcoString {
+        eco_format!("{self}")
+    }
+}
+
+impl IntoAttr for NonZeroI64 {
+    fn into_attr(self) -> EcoString {
+        eco_format!("{self}")
+    }
+}
+
+impl IntoAttr for NonZeroU64 {
+    fn into_attr(self) -> EcoString {
+        eco_format!("{self}")
+    }
+}
+
+impl IntoAttr for f64 {
+    fn into_attr(self) -> EcoString {
+        // HTML float literal allows all the things that Rust's float `Display`
+        // impl produces.
+        eco_format!("{self}")
+    }
+}
+
+impl IntoAttr for PositiveF64 {
+    fn into_attr(self) -> EcoString {
+        self.get().into_attr()
+    }
+}
+
+impl IntoAttr for Color {
+    fn into_attr(self) -> EcoString {
+        eco_format!("{}", css::color(self))
+    }
+}
+
+impl IntoAttr for Duration {
+    fn into_attr(self) -> EcoString {
+        // https://html.spec.whatwg.org/#valid-duration-string
+        let mut out = EcoString::new();
+        macro_rules! part {
+            ($s:literal) => {
+                if !out.is_empty() {
+                    out.push(' ');
+                }
+                write!(out, $s).unwrap();
+            };
+        }
+
+        let [weeks, days, hours, minutes, seconds] = self.decompose();
+        if weeks > 0 {
+            part!("{weeks}w");
+        }
+        if days > 0 {
+            part!("{days}d");
+        }
+        if hours > 0 {
+            part!("{hours}h");
+        }
+        if minutes > 0 {
+            part!("{minutes}m");
+        }
+        if seconds > 0 || out.is_empty() {
+            part!("{seconds}s");
+        }
+
+        out
+    }
+}
+
+impl IntoAttr for Datetime {
+    fn into_attr(self) -> EcoString {
+        let fmt = typst_utils::display(|f| match self {
+            Self::Date(date) => datetime::date(f, date),
+            Self::Time(time) => datetime::time(f, time),
+            Self::Datetime(datetime) => datetime::datetime(f, datetime),
+        });
+        eco_format!("{fmt}")
+    }
+}
+
+mod datetime {
+    use std::fmt::{self, Formatter, Write};
+
+    pub fn datetime(f: &mut Formatter, datetime: time::PrimitiveDateTime) -> fmt::Result {
+        // https://html.spec.whatwg.org/#valid-global-date-and-time-string
+        date(f, datetime.date())?;
+        f.write_char('T')?;
+        time(f, datetime.time())
+    }
+
+    pub fn date(f: &mut Formatter, date: time::Date) -> fmt::Result {
+        // https://html.spec.whatwg.org/#valid-date-string
+        write!(f, "{:04}-{:02}-{:02}", date.year(), date.month() as u8, date.day())
+    }
+
+    pub fn time(f: &mut Formatter, time: time::Time) -> fmt::Result {
+        // https://html.spec.whatwg.org/#valid-time-string
+        write!(f, "{:02}:{:02}", time.hour(), time.minute())?;
+        if time.second() > 0 {
+            write!(f, ":{:02}", time.second())?;
+        }
+        Ok(())
+    }
+}
+
+/// A direction on the X axis: `ltr` or `rtl`.
+pub struct HorizontalDir(Dir);
+
+cast! {
+    HorizontalDir,
+    v: Dir => {
+        if v.axis() == Axis::Y {
+            bail!("direction must be horizontal");
+        }
+        Self(v)
+    },
+}
+
+impl IntoAttr for HorizontalDir {
+    fn into_attr(self) -> EcoString {
+        self.0.into_attr()
+    }
+}
+
+impl IntoAttr for Dir {
+    fn into_attr(self) -> EcoString {
+        match self {
+            Self::LTR => "ltr".into(),
+            Self::RTL => "rtl".into(),
+            Self::TTB => "ttb".into(),
+            Self::BTT => "btt".into(),
+        }
+    }
+}
+
+/// A width/height pair for `<link rel="icon" sizes="..." />`.
+pub struct IconSize(Axes<u64>);
+
+cast! {
+    IconSize,
+    v: Axes<u64> => Self(v),
+}
+
+impl IntoAttr for IconSize {
+    fn into_attr(self) -> EcoString {
+        eco_format!("{}x{}", self.0.x, self.0.y)
+    }
+}
+
+/// <https://html.spec.whatwg.org/#image-candidate-string>
+pub struct ImageCandidate(EcoString);
+
+cast! {
+    ImageCandidate,
+    mut v: Dict => {
+        let src = v.take("src")?.cast::<EcoString>()?;
+        let width: Option<NonZeroU64> =
+            v.take("width").ok().map(Value::cast).transpose()?;
+        let density: Option<PositiveF64> =
+            v.take("density").ok().map(Value::cast).transpose()?;
+        v.finish(&["src", "width", "density"])?;
+
+        if src.is_empty() {
+            bail!("`src` must not be empty");
+        } else if src.starts_with(',') || src.ends_with(',') {
+            bail!("`src` must not start or end with a comma");
+        }
+
+        let mut out = src;
+        match (width, density) {
+            (None, None) => {}
+            (Some(width), None) => write!(out, " {width}w").unwrap(),
+            (None, Some(density)) => write!(out, " {}d", density.get()).unwrap(),
+            (Some(_), Some(_)) => bail!("cannot specify both `width` and `density`"),
+        }
+
+        Self(out)
+    },
+}
+
+impl IntoAttr for ImageCandidate {
+    fn into_attr(self) -> EcoString {
+        self.0
+    }
+}
+
+/// <https://html.spec.whatwg.org/multipage/images.html#valid-source-size-list>
+pub struct SourceSize(EcoString);
+
+cast! {
+    SourceSize,
+    mut v: Dict => {
+        let condition = v.take("condition")?.cast::<EcoString>()?;
+        let size = v
+            .take("size")?
+            .cast::<Length>()
+            .hint("CSS lengths that are not expressible as Typst lengths are not yet supported")
+            .hint("you can use `html.elem` to create a raw attribute")?;
+        Self(eco_format!("({condition}) {}", css::length(size)))
+    },
+}
+
+impl IntoAttr for SourceSize {
+    fn into_attr(self) -> EcoString {
+        self.0
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_tags_and_attr_const_internible() {
+        for elem in data::ELEMS {
+            let _ = HtmlTag::constant(elem.name);
+        }
+        for attr in data::ATTRS {
+            let _ = HtmlAttr::constant(attr.name);
+        }
+    }
+}