HTML may exist without CSS (which would be really unattractive! ), but CSS cannot exist without HTML. The HTML is styled via CSS. The content of a webpage is written in HTML. Consider it similar to a Word document in which you have all of your content (headings, subheadings, paragraphs, bullet points, etc). CSS is the content’s style or look. So CSS is like an editor that tells you what colour, font, and size each piece of text should be, as well as where it should go. CSS can also be used to position elements on the page. Separate files for HTML and CSS should always be used. While it is possible to integrate CSS in an HTML document, it is recommended to maintain it in its own document that the HTML document refers to. https://www.phptpoint.com/html-tutorial/