The World Wide Web
When the Internet and the World Wide Web began over a decade ago, its purpose was to allow government departments and academic institutions to share documents. The computers at that time weren't much good for graphics, so the document were mainly plain text.
The Web today is still very document-centric, in fact, the 'T' in HTML means Text. Sure, we now use pictures, animation, sound and streaming video with varying degrees of success but the Web is primarily a textual medium.
The 'H' in HTML means 'Hyper', which doesn't make much sense without the word 'Text' after it, but it refers to the ability to jump to other text by clicking on a recognisable link. At this point, I'm not defining the term 'recognisable link' – that comes later.
Mark-up, the 'M' in HTML is all about specifying what a particular piece of text does, not how it looks, although its appearance might well change depending on its function. A section of text could be a heading, a list, a link, or just plain body text. Just like the station wagon, 'style' doesn't come into it.
The 'L' means 'Language' and yes, HTML is a language. It's a language that browsers understand and has its own vocabulary and syntaxes just like any other language.
The purpose of HTML is to tell a browser how to display the text. If you want to tell the browser what the text should look like, you should use a 'style sheet'. You see, we are now back to 'design' and 'styling' again.
Unfortunately, before style sheets became widely understood, people tried to style their pages using HTML and found that it didn't do a very good job of it. Then they started using various hacks to adjust spacing and positioning. Then, pages started to break. HTML was never designed to do this and it's no big surprise that it doesn't.