Hypertext Markup Language popularly known as HTML is a popular developer language that has often been argued to be a programming language.
There are hundreds of programming languages and each one of them have their unique instructions and features but before a language can be said to be a programming language, it must be a collection of written symbols in machine and human readable form for the purpose of developing software instructions or script for a computer to understand and carry out.
HTML, on the other hand, as the name implies, is a markup language this is because through HTML alone, developers have been able to use representation of data to create designs, format websites and structure web pages and electronic documents.
A Markup Language is used mostly in creating websites. it is generally used to control the style of display of characters on web pages and athough Markup Languages are limited, they're easy to read and understand.
HTML is one of the simplest developer languages but it gets more complex with each improved form like the HTML5 and in conjunction with some programming languages like JavaScript, it can be used for more complex developments other than just website structuring
Example: HTML5 and JavaScript are often used to display videos and audio.
So, HTML is not a programming language but developers often apply it in conjunction with programming languages.