[css-text] Collapsing whitespace at the end of a line

According to <https://drafts.csswg.org/css-text/#collapse>, whitespace
preceding a segment break is removed.  However, it appears that
browsers instead collapse it to one space.

Here's an example using 'white-space':

<div style="white-space: pre-line;">a
z</div>

(Just in case the email client trims things, there's a space at the
end of the first line, after the "a".)

Here's an example using <br>:

<div style="white-space: pre-line;">a <br>z</div>

In both of these examples, if you highlight the "a" and then drag
slightly rightward, you'll see it highlight a space character as well.
This happens in both Chrome and Firefox.

This sgugests that browsers are not following the "preceding" part of
step 1 in that section, and are instead falling down to step 4, where
runs of spaces are collapsed down to a single visible space.

Is there a particular reason for this?  Should we adjust the spec to
match implementations, or file bugs on implementations to match the
spec?

~TJ