Clarify what 'contentful' means

[noamr]

A shot at better defining what "contentful" means in the context of "first contentful paint". Added "contentful" to the terminology with an attempt at a precise definition with proper references.
Gave clear definitions of images, canvas, text, and paint rects.

(This is my first try at one of these specs so please bear with me...)

@rniwa, @npm1 wdyt?

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Preview | Diff

[npm1]

Thanks for the PR! Left some comments.

[rniwa]

I've gotta say things are looking really good! A few more iterations on this PR should bring a much needed rigor to the definition of contentful. As much as I've been critical of all the phrases in this PR, I can't express enough of my gratitude for all your hard work :) Thanks for all the great work so far & let's keep going with this.

[noamr]

I've gotta say things are looking really good! A few more iterations on this PR should bring a much needed rigor to the definition of contentful. As much as I've been critical of all the phrases in this PR, I can't express enough of my gratitude for all your hard work :) Thanks for all the great work so far & let's keep going with this.

Thanks, it's important for me to hear this, and I appreciate your rigor and the interest in fleshing out the details.

[npm1]

Thanks for working on this! Setting to "Approval" so you're not blocked on me when everyone else is happy with it.

[noamr]

@rniwa any additional comments or can I merge this?

[yoavweiss]

LGTM % nits

[npm1]

Looks great!