[css-align][css-grid] synthesized baselines are insufficiently specified

[MatsPalmgren]

I'm trying to understand what bullet 3 here means:
https://drafts.csswg.org/css-grid/#grid-baselines

What does "failing that" mean there? I read it as, "if there
are no grid items", but I don't know if you mean synthesizing
a baseline from a grid item can fail in some way. Furthermore...

https://drafts.csswg.org/css-align-3/#synthesize-baselines
"To synthesize baselines from a rectangle (or two parallel lines),
synthesize the alphabetic baseline from the lower line and the central
baseline by averaging the positions of the upper and lower lines."

I don't understand what precise code I should write to implement
this paragraph. Does "averaging the positions" mean I should take
the distance between the two lines and divide it by 2 to get
the alignment baseline to use? IOW, the synthesized baseline of
a content box is the center of the content box in the relevant axis?

[MatsPalmgren]

Oh, I think I got what CSS Align is saying now. You're describing
two separate types of baselines in one sentence. To synthesize
an alphabetic baseline - use the lower line. To synthesize central
baseline - use the half-way point between the two lines.

Could you perhaps rephrase it as two separate sentences to
make that clearer?

[tabatkins]

Oh, I see the confusion! Yay for ambiguous parses!

Yeah, I can rephrase that.

[MatsPalmgren]

While you're here, you might want to change the use of "upper" and
"lower" line which doesn't make much sense in a writing-mode world.
I'm guessing it means start-most and end-most in the relevant axis.

[tabatkins]

What does "failing that" mean there? I read it as, "if there
are no grid items",

Yup, you got it. I've made it more explicit.

Could you perhaps rephrase it as two separate sentences to
make that clearer?

I added a comma to separate the two instead, so you can't parse them as one extended clause anymore.

While you're here, you might want to change the use of "upper" and
"lower" line which doesn't make much sense in a writing-mode world.

Yeah, rephrased slightly.