Atom does not have a built-in command for formatting html. However, you can install the atom-beautify package to get this behavior.
- Press CTRL + SHFT + P to bring up the command palette (CMD + SHFT + P on a Mac).
- Type Install Packages to bring up the package manager.
- Type beautify into the search box.
- Choose atom-beautify or one of the other packages and click Install.
- Now you can use the default keybinding for atom-beautify CTRL + ALT + B to beautify your HTML (CTRL + OPTION + B on a Mac).