Section 52.4 Tabs
A tab is the worst sort of whitespace! (See Section 52.3.) There should never ever be any tabs present in any contribution to PreTeXt. The problem: the “width” of a tab is different for each person who opens a file. Two spaces, four spaces, eight spaces, you name it. So if indentation is accomplished with a mix of tabs and spaces, there is a good chance it might look right to you, but looks wrong to many other developers. I hit the tab key on my keyboard all the time. But my editor is set to supply four spaces as a result. See if your editor will behave similarly.