The major difference between HTML and XHTML is that XHTML requires that you be very consistent with your coding, meaning all of it needs to be correct. It requires that your source code be more structured. Using HTML will pretty much guarantee your site won’t look the same in EVERY browser, which to me is very frustrating, especially outdated browsers like Internet Explorer. Sometimes, I’ll look at my (personal) website, not this one, in Firefox, Internet Explorer, and Safari and I’ll see that Internet Explorer is the most off, especially with padding and line height in my CSS. Not that making your site look the same in every browser can’t be done, you just have to add certain codes to your CSS and website document to ensure that it does. However, XHTML seems like a nice solution right now. It’s always worth switching to a markup language that’s an actual upgrade.
CSS (Cascading Style Sheet), a proper definition would be -
a simple mechanism for adding style (e.g. fonts, colors, spacing) to Web documents.
[w3.org]
So, basically, a webpage would be pretty plain and boring without CSS, every site uses it. It also includes the change in link colors, headers, font size, font type, font weight, etc. It’s quite easy to learn how to do. You could actually create an entire web page by just using CSS, which I’ve just recently learned how to do.
Web Standards are – Standardized specifications for Internet markup languages such as HTML, CSS, and XML. Formulated by the W3 organization, these standards enable people to create websites that will work in almost any browser or internet-enabled device, instead of being specific to certain versions of browsers.
Web Standards are significant because it would be pretty limitless if a website only worked in let’s say…Internet Explorer. Not everyone in the world is internet savvy to realize “oh, maybe this site doesn’t work in my browser, but if I download another one maybe it would work”, so it’s important for sites to work in the most popular browsers as Mozilla Firefox and Internet Explorer.
Helpful Sources: About Web Design, WordPress Codex, W3.Org, Internet Glossary