[throwawaykf05]

1. Letting IE stagnate is different from holding back "ideas! concepts!" because Firefox (free software) and Opera were still around to pick up the slack. Sure, it caused (and still does) a lot of pain to web developers to make cross-browser compatible webpages, but that's not the same as "holding back ideas".

2. Note also that vendor prefixes always were and are still a thing, so it's not just IE that had "non-compliant" features. Adding non-compliant features and then working them into the standards seems to be the natural way web technologies advance.

Especially with Opera in mind, I recall many new features being added to browsers regardless of IE's stagnation. Sure, they were not widely used because the then-dominant IE didn't have them, but that didn't prevent Opera and Firefox from adding them. So, unless you conflate "experimentation and advancement" with "widespread adoption", I would disagree that IE held back experimentation and advancement of web technologies.