The secret is: if you use semantic HTML, then they do the work, not you. Their browser does the work, not you. If your pages use semantic HTML, you’re not going to get bug reports saying that your web app doesn’t work in a screenreader, or your buttons don’t work without mouse clicks, or your site doesn’t show anything on a Yoyodyne SuperPhone 3 running FailBrowser, because it will and they will and it will. And using semantic HTML elements is no more effort; it’s just as easy to use
mainas it is to use
div id="main". Easier, even.