In commit 4d49ec8ef we replaced an `@extend .fa-` clause with a
`content: fa-content()` clause.
With the `@extend` clause, the `content:` property appeared wherever the
`.fa-` selector was defined, so we later overwrote it in our `%svg-icon`
selector, which was defined later in the generated CSS.
Defining the property with `content: fa-content()`, on the other hand,
caused the `content:` property to appear wherever we used the mixin with
`@include has-fa-icon`. That meant our `%svg-icon` selector would appear
before it, and would not overwrite it.
We could modify a few things and make the code more complicate in order
to avoid that. In this case, however, it's easier to add an `!important`
flag; after all, it is indeed important that SVG icons have no content
so screen readers don't try to announce illegible characters.
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