First of all, HTML is not used for any design. HTML is merely textual and structural markup, in itself offers next to no design functionality. Imagine HTML as the raw structure of a building. Very bare, minimal and fairly unpleasant to look at. What adds the design layer is CSS, which, to keep to the previously mentioned analogy, would be like a team of interior and exterior designers, with buckets of paint and crazy artistic ideas. Finally, you will have JavaScript which would act as a team of electrical engineers who would put in fancy sliding doors, automatic lightning, smart lock, and whatnot. CSS and JavaScript contribute largely to the design of a web page. HTML is just there to hold it all together.
The below screen-grab illustrates this relationship:
Now, to answer your question though, no, HTML is not only used for web. HTML is an extremely popular language used in websites, but not only. You can use HTML to build eBooks, online magazines, basically any digital publishing. HTML is also very popular in mobile apps for both iOS and Android. Same goes for desktop applications for Windows, Mac and Linux. Even some IOT devices use HTML. So knowing HTML in the modern world is almost a must.
