Most websites nowadays use standard practices of HTML, CSS, and JavaScript so that there sites function and can be used by everyone. So sites don’t use these meaning that some browsers won’t work very well with them. This can cause difficulties navigating and using the site, that is why the W3C decided to come up with the standard practices of site building.
Since HTML is the content, you are putting on a page you must have an understanding of it before you can work on the CSS. The CSS determines how that content will look, so if there is no content you can’t style it. If you have a well written HTML then you will have a better chance at having a page that can be viewed and used on a variety of devices.
In order for the HTML to work it must be correctly ordered and places so that it can be read and understood on all devices. For proper hierarchy you start with your most important aspects and things that may affect the whole page like fonts. Then you will move down in importance to things like the header or title, then to the body, and lastly would be the footer.