Most people who write for the web probably do indeed write in something like markdown, which means they probably sprinkle in a bit of HTML for the parts which markdown doesn't natively support. I imagine a lot of blog posts written in markdown contain a few <table> elements, for example. Anyone writing math content for the web using markdown will have to either write the mathml directly, or write the math expressions in another language and manually compile it to mathml which is copy/pasted into the document. Maybe the CMS they happen to use will some day add native support for compiling latex, but that sounds rather unlikely.