Why not make header id="header" and using a class for span?

Sebastian Finé Hey Sebastien, I may not know all the reasons, but I'll answer your question to the best of my knowledge. I try my to use IDs as sparingly as possible because they have a very high selector specificity which means not many other selectors can override their styles. This being said, if you give a class an ID and months down the line try to change the styles by its class name you may run into an issue where the styles aren't changing and be completely lost as to why they are not. Another thing to mention here, is that there actually could be more than one header element. The header element isn't exactly specific to the header of a page, you could have a header to a specific section as well. One last thing, I would avoid styling the header based on the "top" ID because imagine if someone else contributes to your code and is in your CSS file looking to change a style in the header. They would be looking for a CSS class with the name "header" or something that resembles the page heading. They would not be very likely to check the #top selector. I hope all this makes sense, let me know if you need any clarifications.