Merge branch 'main' into copilot/fix-mathml-font-issue

Chromium maps the math font family to a single font per platform, which
Linux and ChromeOS don't install, so MathML falls back to a text font and
its brackets and large operators don't stretch. Keep the browser's choice
and fall back to the math fonts those systems ship, instead of bundling
a font.

Assisted-by: Claude Code:claude-opus-5-5

web_src/css/base.css

@@ -2,6 +2,8 @@
   /* fonts */
   --fonts-proportional: -apple-system, "Segoe UI", system-ui, Roboto, "Helvetica Neue", Arial;
   --fonts-monospace: ui-monospace, SFMono-Regular, "SF Mono", Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace, var(--fonts-emoji);
+  /* Chromium's "math" is a font Linux and ChromeOS don't ship, so fall back to the math fonts they do */
+  --fonts-math: math, "STIX Two Math", "DejaVu Math TeX Gyre", "Noto Sans Math";
   /* GitHub explicitly sets font names like: "Apple Color Emoji", "Segoe UI Emoji", "Noto Color Emoji", "Twemoji Mozilla";
   Actually "Twemoji Mozilla" emoji font is widely used by browsers like Firefox, Pale Moon, and it is more likely up-to-dated than the system emoji font.
   So not setting emoji font seems to be the best choice, here we just use a non-existing dummy font name and let browsers choose. */
@@ -107,6 +109,10 @@ samp,
   font-size: 0.95em; /* compensate for monospace fonts being usually slightly larger */
 }
 
+math {
+  font-family: var(--fonts-math);
+}
+
 /* there are many <code> blocks in non-markup(.markup code) / non-code-diff(code.code-inner) containers, for example: translation strings, etc,
 so we need to make <code> have default global styles, ".markup code" has its own styles and these styles sometimes conflict.
 TODO: in the future, we should use `div` instead of `code` for `.code-inner` because it is a container for highlighted code line, then drop this ":not" patch */