enhance: allow attribute-less MathML elements and complete the Core allowlist (#39337) (#39340)

Backport #39337 by @nschloe

Follow-up to https://github.com/go-gitea/gitea/pull/38034.

bluemonday only keeps an element without attributes if it was registered
via `AllowNoAttrs`. The MathML rules only used
`AllowAttrs(...).OnElements(...)`, so plain `<mi>x</mi>`, `<mrow>`,
`<msqrt>` and friends were stripped to bare text. Since real-world
MathML is almost entirely attribute-less elements, nearly every formula
collapsed. The existing test missed it because its only case gave every
element an attribute.

Also fills the remaining gaps against MathML Core
(https://www.w3.org/TR/mathml-core/): `rowspan` on `mtd` alongside
`columnspan`, the `maction` element with `actiontype` and `selection`,
and the reserved global attributes `intent` and `arg`.

Tests cover attribute-less round-trips, table cell spans, and `maction`
with `intent`/`arg`.

Co-authored-by: Nico Schlömer <nschloe@users.noreply.github.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/markup/sanitizer_default.go

@@ -70,6 +70,7 @@ func (st *Sanitizer) createDefaultPolicy() *bluemonday.Policy {
 		"mi", "mn", "mo", "mtext", "mspace", "ms",
 		// layout elements
 		"mrow", "mfrac", "msqrt", "mroot", "mstyle", "merror", "mpadded", "mphantom",
+		"maction", // although MDN says "maction" is deprecated, we still need to allow it, otherwise, if it is removed, the layout will be wrong
 		// scripting elements
 		"msub", "msup", "msubsup", "munder", "mover", "munderover", "mmultiscripts", "mprescripts", "none",
 		// tabular elements
@@ -77,10 +78,11 @@ func (st *Sanitizer) createDefaultPolicy() *bluemonday.Policy {
 		// semantic annotations
 		"semantics", "annotation", "annotation-xml",
 	}
+	policy.AllowNoAttrs().OnElements(mathMLElements...) // most MathML elements carry no attributes
 	policy.AllowAttrs("display", "alttext").OnElements("math")
 	policy.AllowAttrs(
-		// global presentation attributes
-		"dir", "displaystyle", "mathbackground", "mathcolor", "mathsize", "mathvariant", "scriptlevel",
+		// global attributes
+		"dir", "displaystyle", "mathbackground", "mathcolor", "mathsize", "mathvariant", "scriptlevel", "intent", "arg",
 		// operator attributes
 		"accent", "accentunder", "fence", "form", "largeop", "lspace", "maxsize", "minsize", "movablelimits", "rspace", "separator", "stretchy", "symmetric",
 		// space and padding attributes
@@ -90,7 +92,9 @@ func (st *Sanitizer) createDefaultPolicy() *bluemonday.Policy {
 		// table attributes
 		"columnalign", "columnlines", "columnspacing", "frame", "framespacing", "rowalign", "rowlines", "rowspacing",
 		// cell attributes
-		"columnspan",
+		"columnspan", "rowspan",
+		// maction attributes
+		"actiontype", "selection",
 		// annotation attribute
 		"encoding",
 	).OnElements(mathMLElements...)