The wordpress Antispam Bee is one of the most popular WordPress plugins to prevent detect and prevent spam postet as blog comments.
One of it’s detection strategies is to embbed a css-based hidden form field. While human users do not see and thus not fill out this field, bots often do. So if Antispam Bee recognizes this field to be submitted with content, the comment is marked as spam.
Unfortunately, many wordpress administrators report that all their comments are marked as spam. All with the spam reason: “Spam Reason: CSS Hack”.
The reason for that is that several plugins and wordpress themes get into conflict with this anti spam techniques. If you do not want or can fix this issue in the code of the extension or theme, the best opportunity is to simply deactivate the extension or switch the theme.
To start digging for the “Spam Reason: CSS Hack” problem’s origin: Start deactivate one extension or theme after the other, send a comment by yourself and check if it was categorized as spam or not.
For me, the plugin “Conditional CAPTCHA for WordPress” was the reason of the problem. Others reported for example the plugin “izioSEO” to conflict with the spam bee.
By the way, you might want to turn off the local spam database check. This might have been messed up if there were a lot of false negative spam detection before.