HTML, CSS, and JavaScript are client-side technologies. These are extremely important for web applications, and a professional Java developer should have some knowledge of them. The developers who are experts in both areas are called full-stack developers these days, though I find the name a bit misleading. A certain understanding is inevitable.
HTML is the textual representation of a structured text. The text is given as characters, as in any text file. Tags represent the structure. A start tag starts with a < character, then the name of the tag, then, optionally, name="value" attributes, and finally a closing > character. An end tag starts with </, then the name of the tag, and then >. Tags are enclosed in hierarchies; thus, you should not close a tag sooner than the one that was opened later. First, the tag that was opened last...