[clreq] Space between characters after joining two lines (#293)

xfq has just created a new issue for https://github.com/w3c/clreq:

== Space between characters after joining two lines ==
## Background of this issue

Something like the following paragraph in an HTML file:

```
Here is an English paragraph
that is broken into multiple lines
in the source code so that it can
more easily read in a text editor.
```

will become:

```
Here is an English paragraph that is broken into multiple lines in the source code so that it can be more easily read in a text editor.
```

when rendered in a web browser, i.e., lines are joined and one space is added between two lines.

However, in CJK text (except Hangul), the spaces are not added. For example:

```
这个段落是呢么长，
在一行写不行。最好
用三行写。
```

becomes:

```
这个段落是呢么长，在一行不行。最好用三行写。
```

when rendered in a web browser, i.e., there is not space after `，` and `好`.

## Question

The CSS Working Group would like to know which characters should not have spaces around them when joining lines.

Here is the current list, categorized using Unicode blocks: https://drafts.csswg.org/css-text-3/#space-discard-set

The interesting questions remaining are:

* [Bopomofo](https://en.wikipedia.org/wiki/Bopomofo) (the spaces should **not** be discarded per https://github.com/w3c/csswg-drafts/commit/e56978ecc12165ab7059902fdaa89230ef2cff53)
* [Yijing Hexagram Symbols](https://en.wikipedia.org/wiki/Yijing_Hexagram_Symbols_(Unicode_block)) / [Tai Xuan Jing Symbols](https://en.wikipedia.org/wiki/Taixuanjing) / [Counting Rod Numerals](https://en.wikipedia.org/wiki/Counting_Rod_Numerals_(Unicode_block))
* [Enclosed CJK Letters and Months](https://en.wikipedia.org/wiki/Enclosed_CJK_Letters_and_Months) / [Enclosed Ideographic Supplement](https://en.wikipedia.org/wiki/Enclosed_Ideographic_Supplement)

Unlike [fantasai](https://github.com/w3c/csswg-drafts/issues/337#issuecomment-612686124), I'm leaning towards not having spaces around all three categories above. WDYT? @w3c/chinese-layout-req-contributors

-----

## See also (CSSWG issues)

* https://github.com/w3c/csswg-drafts/issues/337
* https://github.com/w3c/csswg-drafts/issues/4992
* https://github.com/w3c/csswg-drafts/issues/4993

Please view or discuss this issue at https://github.com/w3c/clreq/issues/293 using your GitHub account