.mw-headline and .mw-editsection are sibling elements; the syntax you're using would
only match if the former was an ancestor of the latter. (Or in other words, <span
class="mw-headline"></span><span
class="mw-editsection"></span> vs <span
class="mw-headline"><span
class="mw-editsection"></span></span>.)
You would normally use the CSS sibling selector: `.mw-headline:hover + .mw-editsection { …
}` (note the '+'), however that is not going to work well here because, to click
on the section edit link, you would have to "un-hover" the headline text
itself.
I think you'll have to use the parent hN elements here, listing them all: `h1:hover
.mw-editsection, h2:hover .mw-editsection, h3:hover .mw-editsection, h4:hover
.mw-editsection, h5:hover .mw-editsection, h6:hover .mw-editsection { … }`.
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Matma Rex