This usually means one or more of your CSS files are incompatible with the minification or merging process. It can also be caused by errors in the CSS’s syntax.
You can check the CSS’s syntax by using an online CSS syntax checker such as csslint.net.
If your syntax is not causing the issue, or you cannot change the file with an issue, you can either
- exclude the problematic CSS files from the minification and merging process.
- or not merge the CSS.
To exclude individual CSS files from the minification and merging process, you can go to WP-Optimize > Minify > CSS and go to the “Exclude the following CSS files from processing” section (for more precise steps on how to exclude your CSS files go here).
To disable merging of CSS files, follow the instructions here.
Posted in: Minification