CSS not() with attribute selector doesn't seem to work - html

Question

I have a very simple selector that works, but when adding it to a :not() it no longer seems to recognize it.
h2:not([random-attribute~="value"] h2){
color: red;
}
[random-attribute~="value"] h2{
color: blue;
}
<div class="content">
<h2>Same valid selector, not working</h2>
<div random-attribute="value">
<h2>Valid selector turned blue.</h2>
</div>
</div>
From what I understand, if you put a valid selector inside the not() you will get any h2 element that is not whatever is inside the parenthesis. This is intuitive.
What isn't intuitive, is that the selector within the not() is valid and works when used alone, but when added to the not() it doesn't seem to work.
Is this not a valid way to write this?

Answer 1

You need to style all h2 element that are descendants of elements that are not [random-attribute~="value"] then style h2 that are.
It doesn't hurt to qualify the selector with a direct child combinator too.
Like so:
*:not([random-attribute~="value"]) > h2 {
color: red;
}
[random-attribute~="value"] > h2 {
color: blue;
}
<div class="content">
<h2>Same valid selector, not working</h2>
<div random-attribute="value">
<h2>Valid selector turned blue.</h2>
</div>
</div>
<h2>some other heading</h2>

Answer 2

You have the syntax wrong for ([random-attribute~="value"] h2) It should just be ([random-attribute~="value"]). See below:
h2:not([random-attribute~="value"]){
color: red;
}
[random-attribute~="value"] h2{
color: blue;
}
<div class="content">
<h2>Same valid selector, not working</h2>
<div random-attribute="value">
<h2>Valid selector turned blue.</h2>
</div>
</div>
You are only supposed to put the given attribute in :not(), not the actual element.

Answer 3

In Selectors Level 3, :not only supports a simple selector argument. That will probably change in Selectors Level 4, but browsers don't support it yet.
The negation pseudo-class, :not(), is a functional pseudo-class
taking a selector list as an argument. It represents an element that
is not represented by its argument.
Note: In Selectors Level 3, only a single simple selector was allowed
as the argument to :not().
Meanwhile, you can rewrite
h2:not([random-attribute~="value"] h2)
as
:root:not([random-attribute~="value"]) > h2,
:root:not([random-attribute~="value"]) > :not([random-attribute~="value"]) > h2,
:root:not([random-attribute~="value"]) > :not([random-attribute~="value"]) > :not([random-attribute~="value"]) > h2
/* ... repeat until you get deep enough */
However, instead of using complicated selectors like that, in CSS it's more natural to let the cascade pick the most specific styles. As kristóf baján recommends, you don't even need :not:
h2 {
/* Default styles */
}
[random-attribute~="value"] h2 {
/* Overriding styles */
}

Answer 4

I think you are making your job a little too complicated... :)
You should just use:
[random-attribute="value"] h2{
...
}
h2 {
...
}
This should solve your problem. The reason behind the fact that it is not working as YOU would expect it to is that the selector inside the not operator is supposed to extend the clarification of the element and not its parent.