Is a good practice to combine multiple css classes when trying to create isolated components? - css

Question

I've been using for some time a "BEM like" syntax in my projects. Recently, I was just re-reading some CSS articles when I saw this: https://en.bem.info/methodology/css/#single-responsibility-principle
Basically, instead of putting all the styles of header__button inside that class, it also relays on styles from button class. Aren't we in this case coupling the element of header with the button class? That means that if in the future, we're gonna change the button class, we also need to remember exactly where we're using this class.
In this example maybe it makes sense because you're trying to have the same styles, but what about a layout component? For instance, let's suppose that I have a Menu class that position some children vertically, and I have a Sidebar class that's also going to apply some style to those children. And we use them like this:
menu.css
Menu {
}
Menu__item {
}
sidebar.css
Sidebar {
}
Sidebar__item {
}
index.html
<div class="Menu Sidebar">
<div class="Menu__item Sidebar__item">
</div>
<div class="Menu__item Sidebar__item">
</div>
</div>
If we don't put all the code about how to position items in the Sidebar class and in the future we change some of this code from Menu, maybe the Sidebar class is going to be broken. In the other case, if we repeat code in both classes Menu and Sidebar we're violating the SRP (single responsibility principle) discussed at the begging of the question. That's what lately, in my projects, I've been favoring code duplication, so I would write all the code needed for a Sidebar into the Sidebar class.
But, what would be the best practice here?

Answer 1

There's also a chapter which partially answers your question: https://en.bem.info/methodology/css/#external-geometry-and-positioning
In your example I'd day Sidebar should be responsible for Menu positioning but it shouldn't know anything about Menu__items. And Menu in its turn should not know anything about its own positioning but should position its items inside.
If you really need to change something in Menu__item positioning when it's inside Sidebar use nested selectors:
.Menu {
}
.Menu__item {
}
.Sidebar {
}
.Sidebar .Menu__item {
}

Answer 2

I think, you should decide, what is it: menu or sidebar, and use one of these classes. If blocks are different a bit, you should use modifiers. If it is impossible to do this with modifiers, use two different classes.
Using modifiers:
// your menu
.menu { }
.menu__item { }
// your sidebar
.menu--some-modifier { }
.menu__item--some-modifier { }
//
.sidebar__menu {
// set some styles (margins, positioning) for .menu
}
Two independent different blocks:
// your menu
.menu { }
.menu__item { }
// your sidebar
.sidebar { }
.sidebar__item { }