Ever since I entered the world of web design, and learned about XHTML, I learn on my own about valid XHTML, and learn how there are guidelines and rules to follow when writing XHTML. This is basically the same as normal HTML, except it further extends the HTML code, and is an application of XML. More about XHTML can be read Wikipedia.
I work really hard to make sure my site is XHTML valid, and that everything I do is XHTML valid. That’s my number one thing to check for when I make webpages. Writing XHTML isn’t hard at all for me, as I’m already used to it. If I’m ever going to learn HTML5 though, that will most likely take time, but that may not happen until the far future. I like XHTML, and for now, I’m sticking with it.