HTML 4.0

The problem with older versions of HTML was such that the language was not adjusted to document formatting. Such characteristics as color or font type had to be added manually to each separate page, which made the process extremely long and complicated.
This issue has been resolved in later versions of HTML. All formatting-related details are replaced from the document into a style sheet. Thus, the control over the document structure remains necessary without interfering into the content.
Thus, it is highly recommended to avoid deprecated tags and use style sheets instead.
XHTML is a newer version of HTML. To start using XHTML it is necessary to get used to putting tags in lowercase.
It is necessary to check out if your HTML documents are valid with HTML 4.01. it can be done with the help of DTD (Document Type Definition).
The HTML 4.01 Strict DTD contains elements and attributes that are accepted by later versions of HTML.
Example:
The HTML 4.01 Transitional DTD contains elements and attributes that are accepted by later versions of HTML and those that are not accepted.
Example:
The HTML 4.01 Frameset DTD contains all above-mentioned elements plus frames.
HTML frames
HTML fonts
HTML styles
Character entities