Is HTML/CSS worth it?

I am learning HTML and CSS to build my website/ future projects, and as a side business to help others build their websites.
Although the languages will not be mainly used in my projects, I don't mind spending time learning them. However, I want to know if they are worth my time. Also, I want advice from someone whose career situation is similar to mine. Even though those languages are not the main ones you use, are they still helpful for you at your job, and to what level?