Short answer: Yes, it's not a good practice.
Long answer: Screen readers do read the ALT
tag; as that's primarily the reason the ALT
attribute was included in the tag. See more.
As of now the Googlebot does not parse CSS and Javascript.. The day may not be far where it might start pulling all the associated files and study the rendering of the page.. I found an interesting article (When Google understands CSS files) that discusses this possibility a while ago. So you are no alone in thinking this way.. :)
Google could start to simply query a website just like a browser does and then parse the object model. How would your pages rank when Google actually "looks" at them?
Spammers generally place many trending keywords in the HTML to help improve their search rankings but smartly hide these keyword dumps from the user by CSS. Googlebot is smart enough to detect such dumps and puts up a penalty for such sites. So YES, Googlebot can understand CSS to a certain extent (and I in no way intent to question the great minds at Google and the capabilities of the Googlebot :) )
So I would be weary and not overdo it..