Earlier quoted context omitted.
In today's webbrowser landscape Safari is the one lagging behind. Firefox is doing very well in regards to implementing web standards while Safari doesn't implement enough to be called up to date or modern.
This is not really true, for what it's worth. The latest releases of all browsers are fairly comparable; Chrome is a bit ahead, and Safari and Firefox mostly trade off against each other: https://caniuse.com/?compare=chrome+103,safari+15.5,firefox+...
Safari doesn't support CSS subgrids, AV1, Push API, HTTP/3, and regex lookbehind.
Except for Push API and video codecs, I see those as major flaws holding back the web.
Firefox can do better at some CSS functionality, with that I can agree.