HTML or HyperText Markup Language is the simplified coding language used by every website on the internet in order to display information to visitors. HTML is a well-established programming language that has been around since the internet was first created.
It can be difficult understanding the difference between HTML and XHTML. More or less, HTML became finalized in the year 2000 while XHTML is still being upgraded. At the moment, XHTML version 1.0 is what’s being used for all websites (including WordPress) while XHTML version 2.0 is still being perfected.
Although you don’t need to understand XHTML to create posts and pages on a WordPress site, it’s important to understand that WordPress uses XHTML as for all of its internal function. Additionally, all themes on WordPress are written in XHTML and so are most plugins. Therefore, if you want to create or modify themes and/or plugins, you’ll need to learn XHTML.
The good news is that XHTML is extremely similar to the original HTML. There are a few grammatical and syntax differences (XHTML is more rigid), and a few of the original HTML tags have been removed from XHTML.
The official documentation for XHTML version 1.0 can be found here.