Your HTML is great! I will just like to see some comments, also with css, you don't have to be lengthy but at least make sure you separate and mention where you are for example:
you could just say <!--- start of brushes ---> code <!--- end of brushes ---> that way, as your apps become more complex all around you're able to quickly skim through and find specific tags that you might need to check so you can target them, or in case of css, because of its unanimity, it will help you separate in blocks of styles, so if you need to change a margin or a font you can get to the line you need way easier, as well as being friendlier if you are working in a team.
Your HTML is great! I will just like to see some comments, also with css, you don't have to be lengthy but at least make sure you separate and mention where you are for example: you could just say
<!--- start of brushes --->
code<!--- end of brushes --->
that way, as your apps become more complex all around you're able to quickly skim through and find specific tags that you might need to check so you can target them, or in case of css, because of its unanimity, it will help you separate in blocks of styles, so if you need to change a margin or a font you can get to the line you need way easier, as well as being friendlier if you are working in a team.