https://drafts.csswg.org/css-text-4/#text-spacing-property

In the current draft, the normal value is defined as:

Specifies the baseline behavior, equivalent to space-start allow-end trim-adjacent.

About the space-start value in this definition, I have proposed to change it to space-first in the following issue:

• [css-text-4] Propose 'text-spacing: space-first' (trim-start-except-first-line) as a normal behavior #2462

Now, I add one more suggestion: change allow-end to trim-end in this definition. So the normal will be equivalent to space-first trim-end trim-adjacent.

I summarize the reason:

• trim-end is typographically better than allow-end in most cases
• trim-end behavior is simpler and needs less overhead than allow-end

trim-end is typographically better than allow-end in most cases

In justified text, the end of lines should be aligned visually. The trim-end is fine for this purpose, and allow-end is not very fine because it makes punctuation at end of lines not alined visually.

Of course, there are cases where allow-end is preferable. Traditionally, in Japanese and Chinese typesetting, a grid-like arrangement with solid setting (no inter-character spacing) is preferable. The trim-end may add extra inter-character spacing when a fullwidth closing punctuation at end of line is trimmed to half-width. So allow-end may be better in this case. However, this will be a less usual case in Web, where the number of characters per line is usually not fixed, and the grid-like arrangement doesn't make much sense.

trim-end processing is simpler and needs less overhead than allow-end

To implement the default behavior (normal), it is important that the processing needs less overhead.

The trim-end processing is simpler, because in this mode, fullwidth closing punctuation can be treated as half-width punctuation plus half-width space which is discarded at line end as like a normal space. On the other hand, allow-end is not so simple because processing needs to be changed depending on the position of the fullwidth closing punctuation in the line, and the processing overhead is not very small.

This is what I have found in my recent experience implementing the text-spacing property in Vivliostyle.js.

FYI, a related blog post on Vivliostyle site: Line end handling has been evolved to allow multiple typesetting options