Unsure if the number for the paragraph spacing (250%) is correct, because 1.5 times 1.5 = 2.25 and not 2.5.
This is already in the old Understanding, but I feel like the 250% doesn't really fit the wording in the SC ("and paragraph spacing is at least 1.5 times larger than the line spacing"). Perhaps I also misunderstand the wording in the SC...?

@JAWS-test I agree the wording for the paragraph spacing is a bit confused/confusing, but the way I understod it (after reading it and re-reading it a few times) is that the 250% comes from having the line height at 150%, plus having the equivalent of a single-spaced extra empty line's worth of space (100%) between the paragraphs, leading to the 150% + 100% = 250%.

made a further attempt to also clarify the paragraph spacing part a bit (the way I understand it, at least).

note that, fundamentally, I also think the normative text of the SC is wrong in how it uses the term "Line spacing (leading)", because it's not saying the leading per se (i.e. the empty space between lines) needs to be a space-and-a-half (as that would equate to line-height: 250% (100% for the text itself, 150% for the leading), but that the "Line height" needs to be 1.5 times the regular font size (100% for the font itself, and leading effectively 50% of the font size)... not going to touch the normative side here. but that is certainly the root cause of the weird confusion ... and only the fact that this is AAA, so not revisited all that often by authors/auditors, has masked this problem for so long.

otherwise, the normative text should really be changed from

Line spacing (leading) is at least space-and-a-half within paragraphs, and paragraph spacing is at least 1.5 times larger than the line spacing.

to

Line height (including leading) is at least space-and-a-half within paragraphs, and paragraph spacing is also at least space-and-a-half.

following up: if the SC indeed intended to mean that leading needs to be 150% (leading to a line height of 250% altogether), then the images/examples provided in the understanding are wrong, as is technique C21 https://www.w3.org/WAI/WCAG21/Techniques/css/C21 (as then that should set line-height: 2.5 rather than line-height: 1.5)

changing this to a draft PR, as the above points (ported back to the issue #2256) are rather fundamental to determine whether or not this is correct (or if indeed it's the technique C21 that's wrong/misleading)

@alastc @awkawk any thoughts/discussion on this would be appreciated - to see if we need to make a normative change at all (and to slip that in in time for 2.2)

Current:

Line spacing (leading) is at least space-and-a-half within paragraphs

Proposed:

Line height (including leading) is at least space-and-a-half within paragraphs

That is a nice editorial patch.

Current:

and paragraph spacing is at least 1.5 times larger than the line spacing.

Proposed:

and paragraph spacing is also at least space-and-a-half.

No!

One (important) benefit of 1.4.8 is to have noticeably more visible white space between paragraphs as compared to lines of text. I don't know that it has to be 2.25. Could be (proposed):

and paragraph spacing is at least twice the line height (that is, double spacing between paragraphs).

and paragraph spacing is at least twice the line height (that is, double spacing between paragraphs).

again, this gets confusing because of the loose use of "spacing" and what that means exactly. do you include the leading of the last line of the first paragraph, and the leading at the top of the first line of the second paragraph, in this concept of spacing? or just the extra bit in between those two leadings?

In essence, is it asking here to - in layperson's terms - have enough space between paragraphs that a line of text (with its ample line height) would fit in there? like there's an empty line between the paragraphs? (generally, that's what's understood when talking about double-spaced lines of text)

(and yes, we still have the problem of what the whole house of cards is actually anchored on)

I think that the understanding changes are ok. I don't think that we are using the term leading incorrectly as in typography it refers to the measurement from baseline to baseline (see https://helpx.adobe.com/indesign/using/leading.html, https://www.indesignskills.com/tutorials/leading-typography/, https://fonts.google.com/knowledge/glossary/line_height_leading).

confusingly, there are conflicting definitions. some refer back to the original meaning (strips of lead being added between the movable type characters), so it's essentially just the inter-line space (between the descenders of one line and the ascenders of the next line). others indeed use leading to mean overall line height (baseline to baseline).

it would probably help if we added an extra diagram as well to illustrate what we/the spec mean when it talks about leading etc

I am glad to read there is no compelling reason to change the phrasing of the SC.

In essence, is it asking here to - in layperson's terms - have enough space between paragraphs that a line of text (with its ample line height) would fit in there? like there's an empty line between the paragraphs? (generally, that's what's understood when talking about double-spaced lines of text)

Yes. Some people need a little more space between lines of text to facilitate more fluid reading. Some people need extra space between paragraphs to help keep their place when reading long tracks of text. Users of screen magnification is one example. If every line is double spaced, paragraph breaks are harder to find (for everyone, but for some people it is really difficult). This SC's requirement to open up paragraph spacing – independent and regardless of the line spacing – is important.

Thanks for 'splaining. I think most of us here are aware of what the SC is trying to achieve...it's more about trying to work out how to more unambiguously convey the concepts here (vis-a-vis confusion on which definition of "leading" is meant, etc).

Anyway, will try to update this some more and then make it ready for review.