Problem scaling background to browser width

[siran]

If it is just about single color rectangles you could also work around the missing stretchy functionailty with some little javascript code. I'm not sure if the standard web designer offers names and code at the website or page head and body sections. But if it does you could check the attached example.

As a summary: It uses a name attached to the horizontal rectangles (the name in concrete is htmlclass=to_full_width which in fact does nothing more than adding a class to the exported html tag). And there is some code added to the website head section, that when exported to the website does something with the rectanges that have the name attached.
In fact it just moves the left edge to the left of the window and changes the width to the width of the window. Okay there is a little bit more to it, but only the fact that when narrowing the browser window to less than the document width, then the code compensates this by doing the calculation based on the document width instead of the browser width.

I hope this babbling didn't distract you from checking it out. And if you have questions on how to use it or it doesn't work with your version (I use v12.6) then just ask again for more explanation or a correction to your version (but you would have to help me out with the exported HTML of your version).