I really don't understand why is Photoshop is favorite tool for webdesign. Adobe it self still call Photoshop as tool primarily for photographers. (see https://creative.adobe.com/en/plans Photoshop is for photographers).
InDesign (by design) should be best tool for that.
Photoshop should be for bitmap, Illustrator for vector, InDesign for layout and putting altogether.
But InDesign's feature "Liquid layout" is not enough useful for web-designers. They need "Responsive layout" which Edge Reflow do well. It now exports really clean HTML & CSS files. Create for prototyping, wireframing,..
Why InDesign doesn't have feature "Generate > Edge Reflow Project" like Photoshop has.
Why is InDesign less supported as tool for webdesign/UI layout/front-end development then Photoshop?
Who knows? What do you thing?
I think that similar problem has every one who want to make progresive formats in book design too. Liquid layout is practiaclly useless. You can't decide about inside margin, proprtion and scale at the same time.