HTML defines what should be on the page (structural structure and content)
HTML documents are the basis of the World Wide Web and are represented by web browsers. HTML is a markup language based on plain text. With HTML (Hypertext Markup Language), these texts are structured in a meaningful and meaningful way, for example by identifying headings, lists, paragraphs, data tables, related areas and important sections as such. HTML is not a programming language, but a text-based description language. Text passages are distinguished using HTML elements. The elements characterize the logical meaning (semantics) of text passages within the website.
CSS defines how the websites should be displayed (formatting and design)
CSS (Cascading Style Sheets) contain the formatting information. These are used for the presentation of the HTML document. The browser displays the result of HTML and CSS. For responsive web design, so-called media queries are an essential prerequisite to allow different designs depending on certain of the available resolution. It makes the most sense to note the CSS specifications in an outsourced stylesheet. Normally, a website consists of several (often hundreds) of pages, all of which should be formatted the same. The advantage of outsourcing is that the design of a website can be changed quickly and easily centrally.