[css-transitions] Clarify behaviors in print documents

[hiikezoe]

Firefox has a bug report that CSS transitions are triggered by @media print style changes. There are two questions;

1. CSS transitions should be triggered by style changes caused by @media print queries?
2. On-going CSS transitions should be printed as it is?

As of now Firefox does both. Chrome doesn't either. Safari does 2 but doesn't 1.

I believe the current Firefox behavior is ideal, but I am quire unsure what the desired behavior is, from the perspective of users, I guess Safari's behavior is reasonable?

We are going to change our behavior to match Chrome one in that bug.

CC @dbaron and @birtles

[birtles]

re: point 2, I notice that CSS Animations has:

Implementations may ignore animations when the rendering medium is not interactive e.g. when printed. A future version of this specification may define how to render animations for these media.

[dbaron]

cc @emilio as well, since he was involved in the linked bug.

My inclination here is that:

1. It would be very strange for a transition that resulted from screen-specific styles to apply when printing.
2. It would be difficult to track whether transitions result from media-specific styles or generally-applicable styles.
3. It seems fine to completely ignore CSS transitions when printing, since there are no dynamic changes happening in the printout.

Given those three things, I think it's probably best to ignore on-going CSS transitions.

That said, I don't think I understand what your question 1 is asking. What do you mean by "style changes caused by @media print queries"?