How can I make a SharePoint site responsive using html?

I will have a test task, and I only know that I’ll have some limited time to design a page in the classic mode of SharePoint Online (with libraries, lists, etc). The task is also to make a page responsive to other devices.

Should I only use HTML responsive meta tags, or media queries in CSS are also required? Is it possible to do it quickly, or it will require a lot of time? Any help is very much appreciated.