feat(csscascade,websem): admit the font-family hint, refuse text-anchor in CSS

The text rung's cascade pair, both measured against Chromium 149 first.

font-family joins the admitted presentation-hint set: the attribute alone
selects the face, an author rule beats it, font-family="" drops like an
invalid declaration, and the property inherits — each pinned by the
precedence law.

text-anchor joins the not-represented refusal list. Chromium consumes it
from the cascade (a rule anchors an attribute-free <text>, and
text-anchor: end beats text-anchor="middle"), but the property is
engine = "gecko" at the Stylo pin, so the servo build has no such
longhand and a sheet declaring one was a silent drop. Same shape as the
gradient rung's stop-color row, one rung later. The attribute spelling
stays admitted and is read by the text compiler.

Author: softmarshmallow

crates/csscascade/src/dom.rs

@@ -729,6 +729,13 @@ fn admitted_svg_presentation_property(local: &str) -> Option<LonghandId> {
         // (measured: `<g font-size="32">` makes a `0.5em` stroke 16px), so
         // dropping it here computed the wrong width from the right document.
         "font-size" => Some(LonghandId::FontSize),
+        // The text rung's addition: the family a run resolves against.
+        // Chromium treats it as a presentation attribute (measured: the
+        // attribute alone selects the face, an author rule beats it, and
+        // `font-family=""` drops so the element renders in the default
+        // family). The consumer matches the computed family against its
+        // declared font environment — a name is not a font identity.
+        "font-family" => Some(LonghandId::FontFamily),
         // The visibility rung's pair. Both are SVG2 presentation attributes
         // whose author-rule precedence is measured (a stylesheet
         // `visibility: visible` overrides `visibility="hidden"` in

crates/csscascade/tests/svg_presentation_hints.rs

@@ -25,6 +25,7 @@ const STANDALONE: &str = r##"<svg xmlns="http://www.w3.org/2000/svg" width="64"
     #transform-none-beats-hint { transform: none; }
     #transform-style-attr-beats-rule { transform: translate(30px, 0px); }
     #transform-webkit-alias { -webkit-transform: translate(30px, 0px); }
+    #family-rule-beats-hint { font-family: monospace; }
   </style>
   <rect id="hint-only" fill="#16a34a" width="8" height="8"/>
   <rect id="named" fill="rebeccapurple" width="8" height="8"/>
@@ -53,6 +54,10 @@ const STANDALONE: &str = r##"<svg xmlns="http://www.w3.org/2000/svg" width="64"
   <rect id="transform-run-together" transform="translate(10-10)" width="8" height="8"/>
   <rect id="transform-webkit-alias" width="8" height="8"/>
   <g color="#d0342c"><rect id="color-basis" fill="currentColor" width="8" height="8"/></g>
+  <text id="family-hint" font-family="Ahem">X</text>
+  <text id="family-rule-beats-hint" font-family="Ahem">X</text>
+  <text id="family-invalid-hint" font-family="">X</text>
+  <g font-family="Ahem"><text id="family-inherited">X</text></g>
   <linearGradient id="gradient-transform-hint" gradientTransform="translate(10 10)"/>
   <linearGradient id="gradient-plain-transform-inert" transform="translate(10 10)"/>
 </svg>"##;
@@ -112,6 +117,25 @@ fn standalone_svg_presentation_hints_enter_below_author_rules() {
         "1",
         "unadmitted presentation attributes must not leak into the cascade"
     );
+    // The text rung's hint: `font-family` is what a run resolves against.
+    // Measured in Chromium: the attribute alone selects the face, an author
+    // rule beats it, `font-family=""` drops to the default family, and the
+    // property inherits.
+    assert_eq!(property(root, "family-hint", LonghandId::FontFamily), "Ahem");
+    assert_eq!(
+        property(root, "family-rule-beats-hint", LonghandId::FontFamily),
+        "monospace"
+    );
+    assert_ne!(
+        property(root, "family-invalid-hint", LonghandId::FontFamily),
+        "Ahem",
+        "an invalid family hint drops like an invalid CSS declaration"
+    );
+    assert_eq!(
+        property(root, "family-inherited", LonghandId::FontFamily),
+        "Ahem",
+        "font-family inherits from the group carrying the hint"
+    );
     // `font-size` is admitted for its *basis*, not because anything paints it:
     // it is what an `em` length resolves against, and Chromium treats it as a
     // presentation attribute. Inherited, so the shape reads the group's.

crates/websem/src/svg.rs

@@ -1065,6 +1065,14 @@ const CASCADE_PROPERTIES_NOT_REPRESENTED: &[&str] = &[
     "stop-color",
     "stop-opacity",
     "color-interpolation",
+    // The text rung's row, the same shape one rung later: Chromium consumes
+    // `text-anchor` from the cascade (measured: a rule anchors an
+    // attribute-free `<text>`, and `text-anchor: end` beats
+    // `text-anchor="middle"`), but the property is `engine = "gecko"` at the
+    // Stylo pin, so the servo build has no such longhand and a sheet
+    // declaring one is a silent drop. The *attribute* spelling is admitted
+    // and read directly by the text compiler.
+    "text-anchor",
     // CSS motion path: measured to translate and rotate an SVG shape — and a
     // whole `<g>` subtree — off its authored position.
     "offset",