What if Firefox and Chromium placed a year-long moratorium on all new browser features unrelated to security, accessibility, and internationalization? Effort not spent on those initiatives could be re-directed towards bugfixes.
Defining the word "major" might be hard but I think it's an interesting idea.
I'm not too worried about including Safari since it could spend those months catching up.
Inspired by a similar article by {{<mention-workitemtype="BlogPosting">}}{{<indieweb-personitemprop="author"name="Peter-Paul Koch"url="https://www.quirksmode.org/about/">}}: {{<cited-workname="Stop pushing the web forward"url="https://www.quirksmode.org/blog/archives/2015/07/stop_pushing_th.html">}}{{</mention-work>}}