What's the point of valid CSS/HTML?

Solution 1:

• Google prefers it so it is good for SEO.
• It makes your web page more likely to work in browsers you haven't tested.
• It allows you to generate or use your page as an XML document.
• It makes you look more professional (to some developers at least)
• Compliant browsers can render XHTML faster than HTML in quirks mode.
• It points out a bunch of obscure bugs you've probably missed that affect things you probably haven't tested e.g. the codepage or language set of the page.

Solution 2:

Because, only by knowing the rule, will you know when to break it.

Using browser-specific extensions isn't bad, but it is something you want to do on purpose, not blindly.

Validation points out the places where you deviate from the standard. Maybe you needed to, maybe you didn't, that decision is yours.

Solution 3:

One reason for having valid HTML is for accessibility. Screen reading software for visually impaired users works much better if the HTML on the page is valid.

Solution 4:

Invalid css/html may become horrible to maintain. Changes may have unwanted effects, which are hard to fix.