XHTML stands for EXtensible HyperText Markup Language. It is a cross between HTML and XML language.
XHTML is almost identical to HTML but it is stricter than HTML. XHTML is HTML defined as an XML application. It is supported by all major browsers.
Although XHTML is almost the same as HTML but It is more important to create your code correctly, because XHTML is stricter than HTML in syntax and case sensitivity. XHTML documents are well-formed and parsed using standard XML parsers, unlike HTML, which requires a lenient HTML-specific parser.
History
XHTML 1.0 became a World Wide Web Consortium (W3C) Recommendation on January 26, 2000. XHTML 1.1 became a W3C Recommendation on May 31, 2001. The standard known as XHTML5 is being developed as an XML adaptation of the HTML5 specification.
Why use XHTML

XHTML was developed to make HTML more extensible and increase interoperability with other data formats. There are two main reasons behind the creation of XHTML:

  • It creates a stricter standard for making web pages, reducing incompatibilities between browsers. So it is compatible for all major browsers.
  • It creates a standard that can be used on a variety of different devices without changes.
Let's take an example to understand it.
HTML is mainly used to create web pages but we can see that many pages on the internet contain "bad" HTML (not follow the HTML rule).
This HTML code works fine in most browsers (even if it does not follow the HTML rules).

For example:

  1. <html>  
  2. <head>  
  3.   <title>This is an example of bad HTML</title>  
  4. <body>  
  5.   <h1>Bad HTML  
  6.   <p>This is a paragraph  
  7. </body>  
The above HTML code doesn't follow the HTML rule although it runs. Now a day, there are different browser technologies. Some browsers run on computers, and some browsers run on mobile phones or other small devices. The main issue with the bad HTML is that it can't be interpreted by smaller devices.
So, XHTML is introduced to combine the strengths of HTML and XML.
XHTML is HTML redesigned as XML. It helps you to create better formatted code on your site.

XHTML doesn't facilitate you to make badly formed code to be XHTML compatible. Unlike with HTML (where simple errors (like missing out a closing tag) are ignored by the browser), XHTML code must be exactly how it is specified to be.