Advantages

HTML is quite easy in its use.
Supported on almost every browser, if not all browsers.
Widely used; established on almost every website, if not all websites.
The syntax is quite flexible.
Very similar to XML syntax, which is increasingly used for data storage.
Its plain text so is easy to edit. Its also fast to download
Free - You need not buy any software.
Easy to learn & code even for novice programmers.

Disadvantages

HTML usually doesn't need JavaScript for validation, Server-script for form execution/operations.
It cannot produce dynamic output alone, since it is a static language.
Its not centralised.
Sometimes, the structuring of HTML documents is hard to grasp.
Very limited styling capabilities
You have to keep up with deprecated tags, and make sure not to use them.
Deprecated tags appear because another language that works with HTML has replaced the original work of the tag; thus the other language needs to be learned .
Security features offered by HTML are limited.
Different vendors added their own custom tags that aren't widely supported if at all by most browsers