This was previously filed as a mail. Reproducing its content here:

Hi,

I checked the following on Safari/Chrome/Firefox for macOS.
http://jsbin.com/nocidux/edit?html,output

<style>
  #tcy {
    border: solid 1px red;
    text-combine-upright: all;
    -webkit-text-combine: horizontal;
  }
</style>
<div style="writing-mode: vertical-rl">
  <span id="tcy"> </span>
</div>

Safari shows the combined white space at the end of the line, but Chrome
and Firefox don't show it.

Should the combined white space at the end of a line be removed?

It looks like Chrome and Firefox remove the combined white space to follow
CSS 3 Text.
https://drafts.csswg.org/css-text-3/#white-space-phase-2

A sequence of collapsible spaces at the end of a line (ignoring any
intervening inline box boundaries) is removed.

However, Safari doesn't remove the combined white space because it should
be considered as the Object Replacement Character
https://www.w3.org/TR/css-writing-modes-3/#text-combine-layout

For other text layout purposes, ... the resulting composition is treated
as a single glyph representing the Object Replacement Character U+FFFC.

I guess that Chrome and Firefox consider White Space Processing as line
breaking and follow:

For line breaking before and after the composition, it is treated as a
regular inline with its actual contents.

I'm not sure if the removing is line breaking because the removing is done
after line breaking.

Which is correct?

Best regards
Yuki Sekiguchi

@fantasai replied:

Wow, I don't know. From the specs you quote, it seems like Safari
is correct, but I'm not sure it makes sense.

It seems to me we did not consider what happens if there is white
space at the start/end of tate-chu-yoko in general, e.g.
text <span id="tcy">tcy </span> text
Maybe such white space should always be trimmed? Probably the best
thing is to combine TCY before white space collapsing, and then
additionally trim the white space at the start/end of the TCY.

What do you think should happen?

Yuki Sekiguchi then replied:

Thank you for answering.

I asked some people who know Japanese typography very well
including Toshi Kobayashi.

Our conclusion is that white spaces at the start/end of tate-chu-yoko
should always be trimmed. Therefore, your suggestion looks great.

I'd like to clarify:

1. The white-space property should prevent the trimming.
2. The Compression Rules should use the trimmed text.
3. The line breaking should use the actual contents of a text combine
   instead of the trimmed text.

If we can change (3) to use the trimmed text, it becomes easier to
implement, but, I think it is needed to be compatible with the current spec
and current implementations.

I could implement the above except white-space support for WebKit. I think
it shouldn't be a problem from the perspective of implementation.

Best regards
Yuki Sekiguchi