I'd say by gut feeling that the rate of change has been reasonably continuous except for the brief period (that felt like it lasted forever) when IE was realistically the only browser worth targeting for a lot of projects. At the time it felt nice that things were "settling down", looking back it was a hideous time when innovation was largely stifled.
Here's a cool resource that plots the history of web tech. It may help you spot a trend:
In university (computer science) we were taught as a rule of thumb (accurate or not) that the half life of computing knowledge is 5 years; i.e. in 5 years only half of the stuff you know now will be relevant. If we're talking pure html / css I'd doubt that's true of the web. Throw in CMS's, webapps, JS frameworks etc. and it probably is a lot closer to true.