User Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:53.0) Gecko/20100101 Firefox/53.0 Build ID: 20170413192749 Steps to reproduce: border-bottom-style:single; Actual results: Error in parsing value for ‘border-bottom-style’. Declaration dropped. book-code.css:99:29 Expected results: should have been accepted
User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:53.0) Gecko/20100101 Firefox/53.0 Firefox: 53.0, Build ID: 20170413192749 I have tested this behavior on latest Firefox (53.0) release and latest Nightly (55.0a1) build, and is reproducible (see screenshot: https://goo.gl/sIZgAt). A "Error in parsing value for ‘border-bottom-style’. Declaration dropped. " warning is displayed in browser console. As Yang, suggested in comment 1, you can use "border-bottom-style:solid;" and it works as expected. No errors are displayed in browser console.
Looks like the console is reporting a correct warning here as per Comments 1 and 2