HTML may be the easiest markup language to learn but there are still some programmers who do some mistakes while writing it, though contemporary Web browsers do really good job to parse them in a good way but there are some error that the browsers cannot handle and the site doesn't look good on your browser.
So for every programmer it becomes a must to validate their markups and stylesheets which will offer much better pages and coding that won't create any problem for user agents as well and they can crawl the pages easily. Those users who are creating take it very seriously to validate their scripting to make sure that each layer of their programming is working fine and not creating any problem of any kind.
In SEO it becomes necessary to make website which will help the crawler to easily crawl the website and not to create any problem for them, in this way one expect better indexing and higher rankings in SERP.
You can validate you HTML on this : http://validator.w3.org/
Just paste you website's url in the search box and hit enter, it will provide you all the info regarding your HTML errors if any with the solutions to resolve them.
Thanks & Regards