Group-income-simple: Explain header styling in CSS section of Style Guide

[taoeffect]

Problem

From this conversation:

It might not be clear to new contributors how and when to use .is-title-X with header tags.

Solution

In our Style Guidelines CSS section, explain:

• How and when to use .is-title-X with header tags
• Why we do it this way

[sandrina-p]

I edited the comment a minute later. Here's an explanation for it. W can add the explanation on our Design System, I think it's the best place for something so specific

[taoeffect]

OK, sure, feel free to include that link so that you don't have to give the long explanation. I think it would be useful to include that link in our Style Guidelines too. Just a sentence would be sufficient if it includes that link.

[sandrina-p]

@taoeffect I did this directly at #793, you can review it there, it's the last commit.