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?