The `fit-content` keyword for the CSS `width` property is currently defined in the CSS Box Sizing Module Level 4 [[ https://drafts.csswg.org/css-sizing-4/#valdef-width-fit-content | Editor's Draft ]], but it is based on the `fit-content()` definition from the CSS Box Sizing Module Level 3 [[ https://www.w3.org/TR/css-sizing-3/ | Working Draft ]] (both of which are standards track).
The change between them is simply from a CSS function to a keyword, but both do the same thing: they invoke the user agent's native content-width calculation. And the reason for the deferment from Level 3 to Level 4 was the overall design of "intrinsic sizing" (of which `fit-content` is a part) rather than significant changes to `fit-content` itself.
`width: fit-content` is [[ https://caniuse.com/mdn-css_properties_width_fit-content | well supported ]] by all modern browsers (minus IE). Most browsers added support for this in the 2015–2017 timeframe, including Firefox, but Firefox still needs the `-moz-` prefix. In other words: it's still just a bit early to use in MediaWiki core or default skins, but plenty mature enough in controlled and limited use cases like TemplateStyles.