HTML has something of a legacy mode, https://en.m.wikipedia.org/wiki/Quirks_mode

Roughly anything before HTML5/4.01 and XHTML renders in quirks mode.

AFAIK there are no legacy CSS properties. Only nonstandard (i.e. prefixed or experimental and never published in finished specs) properties have been deprecated.

Browsers already have some leeway with DOM APIs. https://developers.google.com/web/updates/2017/01/scrolling-...