In modern browsers the CSS `content` property provides alt text support for assistive technology in the form `display / alt text`. This introduces support for legacy browsers that do not support the new syntax.
CSS content properties take advantage of the CSS cascade and browsers ignoring values they don't understand by adding duplicate `content` properties in the form:
{{{#!css
content: display;
content: display / alt text;
}}}
Modern browsers will use the second property, legacy browsers will use the first.
Follow-up to [60885] for #63603.
Props acmoifr, joedolson, jorbin, mydesign78, ov3rfly, peterwilsoncc, presskopp, sabernhardt, siliconforks, swissspidy, threadi, wildworks, wolf45.
Fixes#64350.
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