You simply need to switch from a descendant to a child combinator.
In other words, from this:
:not(.c1) .text
To this:
:not(.c1) > .text
revised fiddle
Your selector...
:not(.c1) .text {
font-weight: bold;
}
is equivalent to this:
*:not(.c1) .text {
font-weight: bold;
}
This selector says:
select an element with class text that is a descendant of any other element, except an element with class c1.
Okay, well, .text is a descendant of a div with class c1 in one instance – so it gets excluded as you intend. But .text is also a descendant of html, body and .container. So the rule fails to work as you expect because it satisfies multiple scenarios.
Instead, try this:
:not(.c1) > .text {
font-weight: bold;
}
This selector says:
select an element with class text when the parent element does not have the class c1.