XML-XHTML-HTML Whats the big deal?

Question

I have become 'fairly proficient' with HTML v4.0.1 transitional.
All my new pages validat w/W3C.org.
Why do I need to learn XHTML or XML?
What's the advantage of using these 'X' languages?
Can one justify the time/effort to transition to one of them?
I will happily go for it if there is some benifit.
Not trying to be offensive.
Just don't want to waste time if it isn't going to help.
Dan H.

Answer 1

A lot of hot air has been expended over the years concerning the benefits of XHTML over HTML and Strict over Transitional - and yes, I hold up my hands as one of the guilty.
About 10 years ago, the W3C decided that HTML was too sloppy as the markup language of web pages. HTML 4.01 was rewritten to conform to the stricter standards imposed by XML. That's what XHTML 1.0 is. It's exactly the same as HTML 4.01, apart from a handful of rules, such as:
tags must be properly nested
all opening tags must have a closing tag or use /> shorthand
all attributes must be in quotes
all attributes must have a value, so checked becomes checked="checked"
all tag names must be in lowercase
If you know HTML 4.01, you can switch to XHTML 1.0 with no difficulty.
At the same time, the W3C decided to deprecate a large number of tags and attributes. Deprecate simply means they are scheduled for removal from a future version of the language, so should be avoided. That's the difference between Transitional and Strict. In Transitional, you are allowed to continue using deprecated tags and attributes. As long as your code is valid, it makes very little or no difference which you use. Aiming for Strict is probably a good idea, though, because the deprecated tags and attributes are features that it's now recommended that you use CSS for. So, if your knowledge of CSS is good, you can probably switch to a Strict DOCTYPE tomorrow. If it's not so good, you probably need the Transitional DOCTYPE. But again, it's whether your code is valid that matters the most.
The W3C's grand plan of moving the web to XHTML hit a massive brick wall when everyone realized that the draft of XHTML 2.0 was not backwards compatible with either XHTML 1.0 or HTML 4.01. The W3C had grandly declared that version 4.01 was the end of the line for HTML, but is has now eaten its words, and work is far advanced on HTML5. The good news is that if HTML5 ever becomes the mainstream markup, it's designed to be backwards compatible with both HTML 4.01 and XHTML 1.0, although all of the deprecated tags and attributes will have gone.
As for XML, it's a very simple language to learn. It's basically just a structured way of presenting data. One of its most common uses is for RSS feeds. The advantage of XML is that it doesn't matter which database the information is stored in, it's presented in a text format that is a universal standard. If you're working with data-driven sites, you should learn XML.