If you are referring to W3C CSS validations, than it’s probably right. Unfortunately the W3C validity does not allow many many properties like SVG fill, rgba backgrounds, background gradients and hundreds of other things. On the other hand every single major browser (including mobile) support these CSS features.
If the search CSS complied the W3C CSS validity, then the plugin most likely would loose all the sales.
If you check any wordpress theme, plugin or anything that has CSS files, you will probably find hundreds of errors with the W3C validator. As an example, check out the AVADA theme (300+ errors) or you can check out facebook as well (40 errors on the login page).
There are many many lines of CSS in the plugin only for the proper compatibilty. Since this is a stock product, it has to work with millions of themes and plugins out there as well as with every major browser.Best,
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