Ideal front-end developer candidate is an expert in CSS and HTML. Is knowledgable of lower level programming paradigms and able to translate those paradigms to client-side scripting such as; JavaScript. So not just a script kiddie who barely knows JavaScript. Will be familiar with the application languages being used and infrastructure being used as well. For example if one is to apply to a shop with a focus on PHP and Wordpress than it would be IDEAL that the front-end developer has completed SEVERAL professional sites using the Wordpress platfor. Lastly, design knowledge IDEALLY formal education and good portfolio.
However, realistically if you HAVE all that it would mean you have both a computer science and design proficiency which is unrealistic. So typically front-end developers will lean to one side or the other with a basic to intermediate understanding of inverse. In my experience good front-end developers lean more towards computer scientist than graphic artist. The problem I have always seen with the art folk whom become front devs is lack of computer science education that directly translates to less than average scripting and structural decisions.
So when a company goes with someone more artsy it is general understand that there is someone there who can deal with JavaScript. On the flip side though when a company goes with the computer science person they loose out on the design side and typically have a dedicated graphic designer to just handle ux and comps. In most circumstances it is a give and take depending what you have already on a team. The thing that remains consistent though Is always HTML and CSS proficiency.
It is rare to find someone who merges the best of the two (computer science and graphic design) but that is the ideal candidate in my opinion.