As I mentioned earlier, one of the big reasons why there is no standard cert is because the field changes too rapidly and erratically for that.
Things like Microsoft, A+, and Cisco certifications have a shelf-life of a few years. Web development stuff can cycle in an out in a matter of a couple months.
Sure, you could have certs for just simple HTML and CSS fundamentals, but any of that could be easily covered by working on pretty much any individual web development project. So, it's kind of moot.
Additionally, there's no one way of doing things. Sure, there are generally accepted approaches for certain things, but that all depends upon the requirements of the problem and the tools/libraries/frameworks that are being used.
Would a certification help employers identify the skill level of a job applicant? Maybe. But, I can usually easily tell what a developer's skill level is just by looking at some of their code. So, if employers don't find certifications useful, it's kind of a moot point to build up a big industry practice around them.
Web development and programming in general is too amorphous to have very many clear baseline standards.