Error log: A potentially dangerous Request.Path value was detected from the client (:).

It happens in 3.9 as well.
Log level  
Error
Short message  
A potentially dangerous Request.Path value was detected from the client (:).
Full message  
System.Web.HttpException (0x80004005): A potentially dangerous Request.Path value was detected from the client (:). at System.Web.HttpRequest.ValidateInputIfRequiredByConfig() at System.Web.HttpApplication.PipelineStepManager.ValidateHelper(HttpContext context)
IP address  
157.166.167.132
Customer  Guest
Page URL  
https://www.bigriverbooks.com/content/jquery-ui-themes/smoothness/data:image/gif;base64,r0lgodlhaqabaiaaaaaaap/yh5baeaaaaalaaaaaabaaeaaaibraa7
Referrer URL  
/bundles/styles/pce-pyc5ahddfjov-672n-kk78xgotjudnersur2pw81?v=flDOXgV8AcC30V1l0gLL23sxtAKOLT-Rku9XCduJNd81
Created on  
10/9/2017 8:28:23 PM

Just use the url data between simple-quotes.

ex: list-style-image: url('data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7');

reference: https://github.com/CSSLint/csslint/issues/442