Target-specific native show rules (#6569)

2 files changed, 413 insertions, 0 deletions

diff --git a/crates/typst-html/src/lib.rs b/crates/typst-html/src/lib.rs
index 60ffa78e..19eb1446 100644
--- a/crates/typst-html/src/lib.rs
+++ b/crates/typst-html/src/lib.rs
@@ -1,8 +1,10 @@
 //! Typst's HTML exporter.
 
 mod encode;
+mod rules;
 
 pub use self::encode::html;
+pub use self::rules::register;
 
 use comemo::{Track, Tracked, TrackedMut};
 use typst_library::diag::{bail, warning, At, SourceResult};
diff --git a/crates/typst-html/src/rules.rs b/crates/typst-html/src/rules.rs
new file mode 100644
index 00000000..f361bfbb
--- /dev/null
+++ b/crates/typst-html/src/rules.rs
@@ -0,0 +1,411 @@
+use std::num::NonZeroUsize;
+
+use ecow::{eco_format, EcoVec};
+use typst_library::diag::warning;
+use typst_library::foundations::{
+    Content, NativeElement, NativeRuleMap, ShowFn, StyleChain, Target,
+};
+use typst_library::html::{attr, tag, HtmlAttrs, HtmlElem, HtmlTag};
+use typst_library::introspection::{Counter, Locator};
+use typst_library::layout::resolve::{table_to_cellgrid, Cell, CellGrid, Entry};
+use typst_library::layout::OuterVAlignment;
+use typst_library::model::{
+    Attribution, CiteElem, CiteGroup, Destination, EmphElem, EnumElem, FigureCaption,
+    FigureElem, HeadingElem, LinkElem, LinkTarget, ListElem, ParbreakElem, QuoteElem,
+    RefElem, StrongElem, TableCell, TableElem, TermsElem,
+};
+use typst_library::text::{
+    HighlightElem, LinebreakElem, OverlineElem, RawElem, RawLine, SpaceElem, StrikeElem,
+    SubElem, SuperElem, UnderlineElem,
+};
+
+/// Register show rules for the [HTML target](Target::Html).
+pub fn register(rules: &mut NativeRuleMap) {
+    use Target::Html;
+
+    // Model.
+    rules.register(Html, STRONG_RULE);
+    rules.register(Html, EMPH_RULE);
+    rules.register(Html, LIST_RULE);
+    rules.register(Html, ENUM_RULE);
+    rules.register(Html, TERMS_RULE);
+    rules.register(Html, LINK_RULE);
+    rules.register(Html, HEADING_RULE);
+    rules.register(Html, FIGURE_RULE);
+    rules.register(Html, FIGURE_CAPTION_RULE);
+    rules.register(Html, QUOTE_RULE);
+    rules.register(Html, REF_RULE);
+    rules.register(Html, CITE_GROUP_RULE);
+    rules.register(Html, TABLE_RULE);
+
+    // Text.
+    rules.register(Html, SUB_RULE);
+    rules.register(Html, SUPER_RULE);
+    rules.register(Html, UNDERLINE_RULE);
+    rules.register(Html, OVERLINE_RULE);
+    rules.register(Html, STRIKE_RULE);
+    rules.register(Html, HIGHLIGHT_RULE);
+    rules.register(Html, RAW_RULE);
+    rules.register(Html, RAW_LINE_RULE);
+}
+
+const STRONG_RULE: ShowFn<StrongElem> = |elem, _, _| {
+    Ok(HtmlElem::new(tag::strong)
+        .with_body(Some(elem.body.clone()))
+        .pack()
+        .spanned(elem.span()))
+};
+
+const EMPH_RULE: ShowFn<EmphElem> = |elem, _, _| {
+    Ok(HtmlElem::new(tag::em)
+        .with_body(Some(elem.body.clone()))
+        .pack()
+        .spanned(elem.span()))
+};
+
+const LIST_RULE: ShowFn<ListElem> = |elem, _, styles| {
+    Ok(HtmlElem::new(tag::ul)
+        .with_body(Some(Content::sequence(elem.children.iter().map(|item| {
+            // Text in wide lists shall always turn into paragraphs.
+            let mut body = item.body.clone();
+            if !elem.tight.get(styles) {
+                body += ParbreakElem::shared();
+            }
+            HtmlElem::new(tag::li)
+                .with_body(Some(body))
+                .pack()
+                .spanned(item.span())
+        }))))
+        .pack()
+        .spanned(elem.span()))
+};
+
+const ENUM_RULE: ShowFn<EnumElem> = |elem, _, styles| {
+    let mut ol = HtmlElem::new(tag::ol);
+
+    if elem.reversed.get(styles) {
+        ol = ol.with_attr(attr::reversed, "reversed");
+    }
+
+    if let Some(n) = elem.start.get(styles).custom() {
+        ol = ol.with_attr(attr::start, eco_format!("{n}"));
+    }
+
+    let body = Content::sequence(elem.children.iter().map(|item| {
+        let mut li = HtmlElem::new(tag::li);
+        if let Some(nr) = item.number.get(styles) {
+            li = li.with_attr(attr::value, eco_format!("{nr}"));
+        }
+        // Text in wide enums shall always turn into paragraphs.
+        let mut body = item.body.clone();
+        if !elem.tight.get(styles) {
+            body += ParbreakElem::shared();
+        }
+        li.with_body(Some(body)).pack().spanned(item.span())
+    }));
+
+    Ok(ol.with_body(Some(body)).pack().spanned(elem.span()))
+};
+
+const TERMS_RULE: ShowFn<TermsElem> = |elem, _, styles| {
+    Ok(HtmlElem::new(tag::dl)
+        .with_body(Some(Content::sequence(elem.children.iter().flat_map(|item| {
+            // Text in wide term lists shall always turn into paragraphs.
+            let mut description = item.description.clone();
+            if !elem.tight.get(styles) {
+                description += ParbreakElem::shared();
+            }
+
+            [
+                HtmlElem::new(tag::dt)
+                    .with_body(Some(item.term.clone()))
+                    .pack()
+                    .spanned(item.term.span()),
+                HtmlElem::new(tag::dd)
+                    .with_body(Some(description))
+                    .pack()
+                    .spanned(item.description.span()),
+            ]
+        }))))
+        .pack())
+};
+
+const LINK_RULE: ShowFn<LinkElem> = |elem, engine, _| {
+    let body = elem.body.clone();
+    Ok(if let LinkTarget::Dest(Destination::Url(url)) = &elem.dest {
+        HtmlElem::new(tag::a)
+            .with_attr(attr::href, url.clone().into_inner())
+            .with_body(Some(body))
+            .pack()
+            .spanned(elem.span())
+    } else {
+        engine.sink.warn(warning!(
+            elem.span(),
+            "non-URL links are not yet supported by HTML export"
+        ));
+        body
+    })
+};
+
+const HEADING_RULE: ShowFn<HeadingElem> = |elem, engine, styles| {
+    let span = elem.span();
+
+    let mut realized = elem.body.clone();
+    if let Some(numbering) = elem.numbering.get_ref(styles).as_ref() {
+        let location = elem.location().unwrap();
+        let numbering = Counter::of(HeadingElem::ELEM)
+            .display_at_loc(engine, location, styles, numbering)?
+            .spanned(span);
+        realized = numbering + SpaceElem::shared().clone() + realized;
+    }
+
+    // HTML's h1 is closer to a title element. There should only be one.
+    // Meanwhile, a level 1 Typst heading is a section heading. For this
+    // reason, levels are offset by one: A Typst level 1 heading becomes
+    // a `<h2>`.
+    let level = elem.resolve_level(styles).get();
+    Ok(if level >= 6 {
+        engine.sink.warn(warning!(
+            span,
+            "heading of level {} was transformed to \
+             <div role=\"heading\" aria-level=\"{}\">, which is not \
+             supported by all assistive technology",
+            level, level + 1;
+            hint: "HTML only supports <h1> to <h6>, not <h{}>", level + 1;
+            hint: "you may want to restructure your document so that \
+                   it doesn't contain deep headings"
+        ));
+        HtmlElem::new(tag::div)
+            .with_body(Some(realized))
+            .with_attr(attr::role, "heading")
+            .with_attr(attr::aria_level, eco_format!("{}", level + 1))
+            .pack()
+            .spanned(span)
+    } else {
+        let t = [tag::h2, tag::h3, tag::h4, tag::h5, tag::h6][level - 1];
+        HtmlElem::new(t).with_body(Some(realized)).pack().spanned(span)
+    })
+};
+
+const FIGURE_RULE: ShowFn<FigureElem> = |elem, _, styles| {
+    let span = elem.span();
+    let mut realized = elem.body.clone();
+
+    // Build the caption, if any.
+    if let Some(caption) = elem.caption.get_cloned(styles) {
+        realized = match caption.position.get(styles) {
+            OuterVAlignment::Top => caption.pack() + realized,
+            OuterVAlignment::Bottom => realized + caption.pack(),
+        };
+    }
+
+    // Ensure that the body is considered a paragraph.
+    realized += ParbreakElem::shared().clone().spanned(span);
+
+    Ok(HtmlElem::new(tag::figure)
+        .with_body(Some(realized))
+        .pack()
+        .spanned(span))
+};
+
+const FIGURE_CAPTION_RULE: ShowFn<FigureCaption> = |elem, engine, styles| {
+    Ok(HtmlElem::new(tag::figcaption)
+        .with_body(Some(elem.realize(engine, styles)?))
+        .pack()
+        .spanned(elem.span()))
+};
+
+const QUOTE_RULE: ShowFn<QuoteElem> = |elem, _, styles| {
+    let span = elem.span();
+    let block = elem.block.get(styles);
+
+    let mut realized = elem.body.clone();
+
+    if elem.quotes.get(styles).unwrap_or(!block) {
+        realized = QuoteElem::quoted(realized, styles);
+    }
+
+    let attribution = elem.attribution.get_ref(styles);
+
+    if block {
+        let mut blockquote = HtmlElem::new(tag::blockquote).with_body(Some(realized));
+        if let Some(Attribution::Content(attribution)) = attribution {
+            if let Some(link) = attribution.to_packed::<LinkElem>() {
+                if let LinkTarget::Dest(Destination::Url(url)) = &link.dest {
+                    blockquote =
+                        blockquote.with_attr(attr::cite, url.clone().into_inner());
+                }
+            }
+        }
+
+        realized = blockquote.pack().spanned(span);
+
+        if let Some(attribution) = attribution.as_ref() {
+            realized += attribution.realize(span);
+        }
+    } else if let Some(Attribution::Label(label)) = attribution {
+        realized += SpaceElem::shared().clone();
+        realized += CiteElem::new(*label).pack().spanned(span);
+    }
+
+    Ok(realized)
+};
+
+const REF_RULE: ShowFn<RefElem> = |elem, engine, styles| elem.realize(engine, styles);
+
+const CITE_GROUP_RULE: ShowFn<CiteGroup> = |elem, engine, _| elem.realize(engine);
+
+const TABLE_RULE: ShowFn<TableElem> = |elem, engine, styles| {
+    // The locator is not used by HTML export, so we can just fabricate one.
+    let locator = Locator::root();
+    Ok(show_cellgrid(table_to_cellgrid(elem, engine, locator, styles)?, styles))
+};
+
+fn show_cellgrid(grid: CellGrid, styles: StyleChain) -> Content {
+    let elem = |tag, body| HtmlElem::new(tag).with_body(Some(body)).pack();
+    let mut rows: Vec<_> = grid.entries.chunks(grid.non_gutter_column_count()).collect();
+
+    let tr = |tag, row: &[Entry]| {
+        let row = row
+            .iter()
+            .flat_map(|entry| entry.as_cell())
+            .map(|cell| show_cell(tag, cell, styles));
+        elem(tag::tr, Content::sequence(row))
+    };
+
+    // TODO(subfooters): similarly to headers, take consecutive footers from
+    // the end for 'tfoot'.
+    let footer = grid.footer.map(|ft| {
+        let rows = rows.drain(ft.start..);
+        elem(tag::tfoot, Content::sequence(rows.map(|row| tr(tag::td, row))))
+    });
+
+    // Store all consecutive headers at the start in 'thead'. All remaining
+    // headers are just 'th' rows across the table body.
+    let mut consecutive_header_end = 0;
+    let first_mid_table_header = grid
+        .headers
+        .iter()
+        .take_while(|hd| {
+            let is_consecutive = hd.range.start == consecutive_header_end;
+            consecutive_header_end = hd.range.end;
+            is_consecutive
+        })
+        .count();
+
+    let (y_offset, header) = if first_mid_table_header > 0 {
+        let removed_header_rows =
+            grid.headers.get(first_mid_table_header - 1).unwrap().range.end;
+        let rows = rows.drain(..removed_header_rows);
+
+        (
+            removed_header_rows,
+            Some(elem(tag::thead, Content::sequence(rows.map(|row| tr(tag::th, row))))),
+        )
+    } else {
+        (0, None)
+    };
+
+    // TODO: Consider improving accessibility properties of multi-level headers
+    // inside tables in the future, e.g. indicating which columns they are
+    // relative to and so on. See also:
+    // https://www.w3.org/WAI/tutorials/tables/multi-level/
+    let mut next_header = first_mid_table_header;
+    let mut body =
+        Content::sequence(rows.into_iter().enumerate().map(|(relative_y, row)| {
+            let y = relative_y + y_offset;
+            if let Some(current_header) =
+                grid.headers.get(next_header).filter(|h| h.range.contains(&y))
+            {
+                if y + 1 == current_header.range.end {
+                    next_header += 1;
+                }
+
+                tr(tag::th, row)
+            } else {
+                tr(tag::td, row)
+            }
+        }));
+
+    if header.is_some() || footer.is_some() {
+        body = elem(tag::tbody, body);
+    }
+
+    let content = header.into_iter().chain(core::iter::once(body)).chain(footer);
+    elem(tag::table, Content::sequence(content))
+}
+
+fn show_cell(tag: HtmlTag, cell: &Cell, styles: StyleChain) -> Content {
+    let cell = cell.body.clone();
+    let Some(cell) = cell.to_packed::<TableCell>() else { return cell };
+    let mut attrs = HtmlAttrs::default();
+    let span = |n: NonZeroUsize| (n != NonZeroUsize::MIN).then(|| n.to_string());
+    if let Some(colspan) = span(cell.colspan.get(styles)) {
+        attrs.push(attr::colspan, colspan);
+    }
+    if let Some(rowspan) = span(cell.rowspan.get(styles)) {
+        attrs.push(attr::rowspan, rowspan);
+    }
+    HtmlElem::new(tag)
+        .with_body(Some(cell.body.clone()))
+        .with_attrs(attrs)
+        .pack()
+        .spanned(cell.span())
+}
+
+const SUB_RULE: ShowFn<SubElem> = |elem, _, _| {
+    Ok(HtmlElem::new(tag::sub)
+        .with_body(Some(elem.body.clone()))
+        .pack()
+        .spanned(elem.span()))
+};
+
+const SUPER_RULE: ShowFn<SuperElem> = |elem, _, _| {
+    Ok(HtmlElem::new(tag::sup)
+        .with_body(Some(elem.body.clone()))
+        .pack()
+        .spanned(elem.span()))
+};
+
+const UNDERLINE_RULE: ShowFn<UnderlineElem> = |elem, _, _| {
+    // Note: In modern HTML, `<u>` is not the underline element, but
+    // rather an "Unarticulated Annotation" element (see HTML spec
+    // 4.5.22). Using `text-decoration` instead is recommended by MDN.
+    Ok(HtmlElem::new(tag::span)
+        .with_attr(attr::style, "text-decoration: underline")
+        .with_body(Some(elem.body.clone()))
+        .pack())
+};
+
+const OVERLINE_RULE: ShowFn<OverlineElem> = |elem, _, _| {
+    Ok(HtmlElem::new(tag::span)
+        .with_attr(attr::style, "text-decoration: overline")
+        .with_body(Some(elem.body.clone()))
+        .pack())
+};
+
+const STRIKE_RULE: ShowFn<StrikeElem> =
+    |elem, _, _| Ok(HtmlElem::new(tag::s).with_body(Some(elem.body.clone())).pack());
+
+const HIGHLIGHT_RULE: ShowFn<HighlightElem> =
+    |elem, _, _| Ok(HtmlElem::new(tag::mark).with_body(Some(elem.body.clone())).pack());
+
+const RAW_RULE: ShowFn<RawElem> = |elem, _, styles| {
+    let lines = elem.lines.as_deref().unwrap_or_default();
+
+    let mut seq = EcoVec::with_capacity((2 * lines.len()).saturating_sub(1));
+    for (i, line) in lines.iter().enumerate() {
+        if i != 0 {
+            seq.push(LinebreakElem::shared().clone());
+        }
+
+        seq.push(line.clone().pack());
+    }
+
+    Ok(HtmlElem::new(if elem.block.get(styles) { tag::pre } else { tag::code })
+        .with_body(Some(Content::sequence(seq)))
+        .pack()
+        .spanned(elem.span()))
+};
+
+const RAW_LINE_RULE: ShowFn<RawLine> = |elem, _, _| Ok(elem.body.clone());