To make text on a Medium post into a header or sub-header:
Select the text and click the large or small T icon from the pop-up menu. Click it again to remove formatting.
Header shortcut: ⌘ + Alt + 1 / Ctrl + Alt + 1
Subheader shortcut: ⌘ + Alt + 2 / Ctrl + Alt + 2
Select the text and click the T icon from the bottom menu to format it as a header. Click it again to change it into a subheader. Clicking the button again will remove the formatting entirely.
Select the text and click the T icon from the bottom menu to format it as a header. Click it again to change it into a subheader. Clicking the button again will remove the formatting entirely.