Fix code-line-numbers links having no accessible name#14662
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Skylighting's numberSource line numbering emits an empty <a> per line, with the visible number drawn purely via a CSS counter on ::before. Screen readers and axe-core see a focusable link with no discernible text (WCAG 2.4.4/4.1.2). aria-hidden isn't an option since these anchors are real, focusable fragment links, and aria-hidden on a focusable element is itself an axe violation. Instead, set aria-label to the visible line number, computed from any counter-reset offset so custom startFrom values stay correct. Implemented as a postprocessor pass in htmlFormatPostprocessor, which revealjs's htmlFormatExtras also inherits, so one pass covers both.
…14655) Verified against real axe-core 4.10 in a browser: the html fixture (numbered code, no other violation source) now reports zero axe-core violations, and the revealjs fixture's report has no violation targeting a code line-number anchor (only the pre-existing, unrelated slide-menu-button/viewport-meta violations remain, tracked separately).
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Skylighting's
numberSourceline-numbering emits an empty<a href="#cbN-n"></a>as the first child of every code line span, drawing the visible number purely via a CSS counter on::before. Screen readers and axe-core see a focusable link with no discernible text (WCAG 2.4.4/4.1.2link-name).Fix
Set
aria-labelon each line anchor to its visible line number, computed from anycounter-reset: source-line Noffset so customstartFromvalues stay correct. Implemented as a postprocessor pass inhtmlFormatPostprocessor(src/format/html/format-html.ts), which revealjs'shtmlFormatExtrasalso inherits, so one pass covers bothhtmlandrevealjs.aria-hiddenisn't an option here: these anchors are real, focusable fragment links, andaria-hiddenon a focusable element is itself an axe violation.Verified against real axe-core in a browser, now codified as a playwright regression test. On revealjs, the axe report still shows a pre-existing
link-nameviolation from the deck's.slide-menu-buttonchrome, unrelated to this issue and out of scope here.Fixes #14655