Browsers should adhere to standards and not enhance without changing the standard first.
Not quite what I meant. They should adhere to any existing standards, but can improve how some things are handled. A page written for Firefox should render the same in any other browser. Rendering without 300MB of ram per byte of page code would be a nice enhancement though.
If one browser does not try something new, there is no reason for the others to catch up. There's less incentive for improvement and a great deal less of a web site. I'm pretty sure that JS did not come about because all the standards were written before the first browsers began to adopt it, nor were HTML and CSS fully written out before browsers began to use them.