You will need HTML, CSS, and JavaScript skills, as well as an understanding of the basics of these technologies. For example, in JavaScript, you need to understand the prototype chain and ES6 features; in CSS, you need to understand what relative, absolute, and fixed means for position property, and how float works and when you need to use clear. On top of that, you need to master several frameworks, for example, Bootstrap, Backbone.js, React, Angular, or Vue.js.

Besides that, you also need to understand the way browsers work and how the JavaScript engine works, because it will help you understand how your frontend code is executed.