CSS selector

I think you need to think about the hierarchy of what you are trying to define as you define things. If "C" is what you are calling this section and it is unique, then "C" should be your ID as opposed to your class. The id in HTML is a unique identifier of a particular element and is not reused on the page, whereas in your case, a class of "C" is a class that can be re-used on multiple elements on your page. So if "C" should be a unique value and a section "ID" is just used to name all sections "A-Z" then I would reverse what you have today. It could also simplify your CSS to look for just section.C instead of section#ID.C.

It all depends on your objectives and the weight you wish to give to your selector (it's in French but very well done https://www.zonecss.fr/cours-css/poids-des-selecteurs-css.html)

Given that your tag uses an identifier, and that there will only be one in the page, chances are that a simple call to this identifier will suffice (unless you're pooling CSS style sheets and proposing a different rendering according to context and zone).

So these selectors are all possible

#ID
.C
#ID.C
section#ID
section.C
section#ID.C
...

However, I'd advise you to use only lowercase letters to values your class and id attributes... so it would be preferable to use :

<section id="id" class="c">

I guess: section#ID.C

Hosun