[selectors] :in-viewport() psuedoclass for scroll-based animations & lazy-loading

Yestarday, Amelia Bellamy-Royds tweeted:
*I also want an :in-viewport psuedoclass for scroll-based animations &
lazy-loading.*
(see tweet: https://twitter.com/AmeliasBrain/status/739865073745833984 )

Today I want to know your opinion about a new css pseudoclass
:in-viewport()
where the value inside the function () defines a viewport area.
The pseudoclass rule will be applied while any part of the element is
inside this area

value for :in.viewport()
[ <length> | <percentage> | auto]{1,4}
The value may be negative.

For example:

.el {
opacity:0;
transition: .5s;
} .el:in-viewport(10vh 0) {
opacity:1;
}


​

Could declared exclusion areas inside :in-viewport()?

:in-viewport(0 / 30vh 0) {}


​

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