Sure, AJAX is cool. However, devs would not be allowed to release into the wild if the experimental features were not availalable in released browsers. That whole -strict, -transitional type stuff is just another example. Extreme specific personal reco would be to only allow experimental features available to devs that must compile their own browser from source. this would prevent the vast majority of users from ever seeing those experimental features in the wild. Devs could still dev against and find the bugs, but they would also be less willing to make the experimental features requirements on their product.
That isn't remotely the case. Plenty of experimental / one-vendor features have been released and later received wider adoption without fundamentally breaking the user's experience.
I.e. before we had indexeddb, there was websql. The whole PWA slew of features (I.e. service workers) can and are used without breaking user experience.
There can be bad implementations too- youtube's awful performance due to their early version of the web component spec comes to mind- but these features arent really akin to the olden days of ActivX / IE dominance, since they are single purpose and not opening up an entire foreign interface.
