HTML is a markup language. It stands for Hyper Text Markup Language. HTML is the building block of a webpage. CSS is a styling language. CSS means Cascading Style Sheets. CSS manages how a building block (HTML) will appear on a webpage i.e., positioning, colouring, sizing etc of different elements (sections).
If a web page was a house, HTML would be the building blocks, CSS would be the paint and decorations and JavaScript would be the electrical wiring.
Have a look at your question. It is HTML which tells the browser to show “How do you convert HTML to CSS?” on the webpage along with other words like “Request“, “Follow“, “Comment“, “Share“, etc. but it is CSS which tells the browser that the question title should be bold and its size should be Xpx (pixels). The “Topics” must appear before the title and must have a background of XXXXXX colour.