[css-transitions]: Add keyword for adding transitions to existing ones

[SetTrend]

Current Situation

Currently, in the following example, the second rule will overwrite the first one:

transition: opacity 400ms ease;
transition: left 350ms ease;

So, only transition: left 350ms ease; will be in effect. transition: opacity 400ms ease; will be ignored

Desired Situation

Please add an add keyword to the different transition properties (incl. the shorthand property), so existing transition rules will remain intact while new transitions may be added independently from existing transitions.

Example

transition: opacity 400ms ease;
transition: add left 350ms ease;
            ‾‾‾

… would yield the same result as:

transition: opacity 400ms ease
          , left 350ms ease;

[birtles]

I think this use case applies to many CSS properties, particularly list-based properties. See the discussion in #1594. If you don't mind, I'd suggest closing this issue in favor of that one.

[tabatkins]

Yup, this is a more general issue we've never successfully solved (and trying to solve it for just this case isn't any simpler than solving it for everything). So, closing as dupe.

[SetTrend]

Thanks for pointing me!