HTML, JavaScript and CSS are binding agents and very basic to web presentation. Eventually it all comes back to these 3 items. Without them then content can't be easily viewed in a browser.
CMS focus more on content and use templates to present the content to the user. The templates merge content (text/data/etc) with HTML, JavaScript and CSS to create the output.
Depending on your desired outcome, you may not need to know a lot about HTML, JavaScript of CSS (or PHP which WordPress uses), but it can help if you know something.
Take care,