Use of a stylesheet is required. Depending on the context, it may be a price, a parameter, or a person. A forgotten tag, or a an attribute without quotes makes the file unusable, while in HTML such practice is often explicitly allowed, or at least tolerated. Also link abilities are more powerful. They consist of elements and entities.
Elements are markup tags similar to HTML tags.
It has been used since 1990. It has gone through several revisions, and is now at version 4.01. HTML will not go through another revision, except as an application of XML, i.e. XHTM 1.0. With HTML, authors had a fixed set of elements to use, with no variation. New tags and attributes can be defined and added to those that already exist, making possible new ways to embed content and programming in a Web page.