If you're going to be purely developing FE, here's what I recommend:
Aside from the obvious couple: HTML(5)/CSS(3), a front-end developer should learn the following:
- JavaScript ES6 (ECMAScript 2015) + Babel (to expend the compatibility of ES6 onto non-compatible browsers by transpiling the code into ES5)
- Unit and Integration tests
- RWD (either mobile-first, desktop-first or element-first)
- Some eventual libraries (e.g.: React, jQuery, ThreeJS, D3, frameworks (e.g.: Angular 2, VueJS, EmberJS)
- SEO, not mandatory but extremely useful since it will get you to learn how to build fast, lightweight and efficient front sides of webpages/webapps.
- AJAX
- Bundlers like Webpack, Browserify or others
- Package managers like NPM, Yarn, JSPM, Bower or Brew.