Jeff Croft argues that because the web standards movement did such a good job, the "HTML and CSS guru" of old is a commoditized job. "Diversify or die", he says, meaning level up your abilities in other web tech to remain employed.
I agree with the sentiment. Web standards have become a thing to be thankful for, not a scrappy underdog. Not that we shouldn't keep a keen eye out for wrong turns.
But I don't think HTML/CSS people are at too much risk. If anything, I've experienced these people being hard to find and in short supply. As Jeff suggests, sprinkle in some fancy modern tools and you're pretty set.