I remember that when I changed from desktop development to website development, I spent at least a month to get familiar to HTML elements, i.e, the tags. and CSS.
Of course for PHP, maybe the official website http://www.php.net/manual/en/getting-started.php
looks more comprehensive.
HTML5 has many special applications, especially some graphic applications such as Canvas, SVG, or WebGL which are used to mainly deal with drawing or game design. They are kind of cuttiing-edge, if you are interested, you can check them out, too.