There are, IMHO, “a whole bunch of extremely good reasons why” it is best:   (a) to determine what you want to generate before you generate it; and (b) to use a templating tool to actually generate the HTML or anything-else.

One of these reasons is “separation of concerns.”   If you do it “the other way,” the day will come when someone wants you to make some page-output change, e.g. so that the page looks noticeably-different in a particular case ... and, perhaps obviously (if you’re very lucky) but perhaps quite subtly, the answers start coming out wrong.   And the logic by that time is so danged jumbled-up that you wish you could rewrite it but in the meantime that you didn’t have to touch it.