I never understood why would somebody limit him/herself unless it's for market positioning.
I’d argue that to be a high quality, effective and fast front-end developer these days you need most of:
- deep knowledge of HTML5
- deep knowledge of CSS layout and text standards
- good knowledge of 5+ different browser families and their versions going back 5 years
- good knowledge of accessibility best practise and the various groups of assistive technology
- good knowledge of the rendering pipeline in multiple different browsers, and the ability to write code that will not cause jank in the user experience
- ability to set up and maintain a build chain
- an eye for design, and the ability at least to talk to designers about implementation problems
It’s absolutely possible to have all of these things, but is it possible to have them all along with the same high quality equivalents on the backend? Doubtful.