There are a couple of problems with what you're after. First is that HTML and CSS are open standards, so there's no central authority on them that can confer credible certification. The closest you could probably come is this:
from the World Wide Web Consortium, which is a body that facilitates the development of the HTML and CSS standards.
Second problem is that, as you know, HTML is not a programming language, but rather a markup language; therefore, most anyone who can use a text editor can learn it pretty easily. Also, there are tons and tons of free HTML tutorials on the web. It's not like learning a true programming language, where you need to understand logic, looping, object-oriented constructs, etc.
So. Having said that, and having no hard data to back up the following statements, my sense is that a) a job as an HTML/CSS trainer would pay less than your current job, and b) any certification that you could get would have little value in helping you find such a position.
Do you have other skills that you've acquired tangentially to your web design work, such as graphic design with Photoshop, or web programming with any of ColdFusion, ASP, ASP.NET, PHP, JSP, etc.? Those skills are much harder to acquire via self-study and I think would be much more likely to be in demand for training and would pay better.