Validating HTML Code
Validating your HTML code to follow the W3C standards and keeping it valid can be, without a doubt, a gigantic pain in the ass. So, why should you! After all, many sites which don't use valid HTML have no problem ranking highly. Google, a rather ironic example, doesn't even come close to validating.
Code which is be broken to the point only Internet Explorer will render it (^_^) and kills browsers is a bad thing. At some level, the search engine will give up, stick a fork in your site and stick it at the bottom of the pile of search results. The law of diminishing returns seems to kick in after most browsers have no trouble rendering a page. The prize for have valid code has never been established to be much more than a possible slight increase over similar sites which don't have valid code.
It can't hurt. Check out the W3C's HTML Validator and show your support for standards.
Character Set
Usually, the search engine spider can work out what language and text encoding your page is. However, getting your page listed under ancient egyptian hieroglyphs might be a little embarrassing and lose you some page views when you're really a page in English with a egyptian hieroglyph lesson on it. Save yourself the embarrassment and set the following tag which is put in the <head> section.
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII" />
This is more important in languages with large character sets and multiple popular ways to encode like Japanese and Chinese. If the user's browser is set to default to another text encoding they just see garbage. See W3C for more information.