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

This is all software, so nothing's impossible if we're creative enough, but making a pseudo class depend on layout (and scrolling) goes against the grain of how CSS works.

The styles you apply when that selector matches can change the layout, which can change whether the selector matches. This kind of loops between the styling layer and the layout layer are really troublesome.

I know there are tons of use cases for something like that, and LG was suggesting something along these lines earlier as well to deal with scroll effects on round displays, but I don't think this approach will actually work.

 - Florian

> On Jun 14, 2016, at 09:25, Rachel Nabors <rachelnabors@gmail.com> wrote:
> 
> I would like to vouch for this. I already use waypoints.js to manually track everything that is in view so I can pause/remove animations that aren't visible. I do this for performance reasons and others do, too. It's in the wild already. Would love to see it official.
> 
> 
>   
> Rachel Nabors
> Web Animation Engineer
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> On Tue, Jun 7, 2016 at 5:40 AM Kseso <kseso9@gmail.com <mailto:kseso9@gmail.com>> wrote:
> 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 <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;
>  }
> 
> <Gráfico1.jpg>
> ​
> 
> Could declared exclusion areas inside :in-viewport()?
> 
> :in-viewport(0 / 30vh 0) {}
> 
> <Gráfico2.jpg>
> ​
> 
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