The #CSS spec has an example:
> setting alt to the empty string can avoid reading out the decorative element
https://drafts.csswg.org/css-content/#example-954890b1

The grammar is:

`[ <content-replacement> | <content-list> ] [/ [ <string> | <counter> ]+ ]?`

Which indicates to me that a `/ ""` is valid.

So is this a #Firefox and #Safari bug bug? 
My thought is "yes". If they think the alt text is invalid, they shouldn't drop the content, they should display it without the alt.

Thoughts? 

 The Gecko bug is https://bugzilla.mozilla.org/show_bug.cgi?id=1818232
The Safari bug is https://bugs.webkit.org/show_bug.cgi?id=159022 - from 2016!

It seems weird to me that they both support content - but error out on the alt. Surely this should be graceful degradation?