This patch is an extension of bug#71605, and the first place to 
explicitly use the new variable-pitch support for 
'visual-wrap-prefix-mode'. While implementing this, I found two small 
bugs in the new 'visual-wrap-prefix-mode' code:

1. When setting the min-width for the first line prefix, we should use 
'add-display-text-property' so as not to clobber other display properties.

2. My attempts to be "helpful" by special-casing wrap-prefixes of all 
spaces ended up just interfering with more complex cases (like SHR), so 
I removed it. The code is now simpler (one fewer condition) and just 
works more smoothly overall.

There's one limitation to this patch though: since SHR uses absolute 
pixel-widths for indenting internally, things can look mis-indented if 
you scale the text in the buffer. However, SHR has exactly the same 
issue when *not* using 'visual-wrap-prefix-mode', so it's really just a 
more-general bug in SHR. (It'd be nice to fix that, but I'd have to get 
a better understanding of how indentation and <table> elements interact.)

Attached is a test HTML page that shows off the indentation. You can see 
the results by running:

   emacs -Q --eval '(progn (setq shr-fill-text nil) (eww "test.html"))'

(And also compare to the default behavior where 'shr-fill-text' is non-nil.)