The difference between HTML and XHTML
1. XHTML elements must be nested correctly
2. XHTML elements must be closed, and empty tags must also be closed. Such as

3. XHTML elements must be lowercase

4. XHTML documents must have a root element

5. XHTML attribute names must be lowercase, attribute values ​​must be in quotation marks, and attributes cannot be abbreviated. For example:
6. XHTML uses the id attribute instead of the name attribute.

Note: An extra space should be added before the "/" symbol to make your XHTML compatible with today's browsers

7. Language attribute (lang) The lang attribute applies to almost all XHTML elements. It defines the type of language used for the content inside the element. If you use the lang attribute in an element, you must add additional xml:lang, like this:

Three XML document types of XHTML 1.0
XHTML 1.0 specifies three XML document types to correspond to the above three DTDs.

Use in situations where clean markup is needed to avoid clutter in performance. Please use it with Cascading Style Sheets.

Use when you need to take advantage of the presentational features of HTML and when you need to write XHTML for browsers that do not support cascading style sheets.

Used in this situation: When you need to use HTML frames to split the browser window into two or more frames.

HTML4 and HTML5

HTML5 is very popular recently. Let’s take a look at some of the differences between HTML5 tags and HTML4 tags:
1. Changes in concepts:

HTML5 focuses on content and structure, but not on performance