From the Sublime Text 3 docs: Auto complete shows the completion popup as you type, so you can fill in long words by typing only a few characters. It's enabled by
Additional ways to set the syntax to HTML without saving first are:
- Open the command palette and select Set Syntax: HTML
- Select View > Syntax > HTML from the menu
- The bottom-right of the window will say Plain Text for a new file; clicking it opens a syntax menu similar to the above one where you can select HTML
- Not Only HTML you can also select others Languages like CSS or javaScript.