Update _screen-reader.scss (#22154)
Update _screen-reader.scss (#22154)

* Update _screen-reader.scss

Small improvements that could be important:
* `clip` [is deprecated](https://www.w3.org/TR/css-masking-1/#clip-property). Adding `clip-path` as progressive enhancement; the shorter notation came from @ryuran 's [suggestion](https://twitter.com/ryuran78/status/778943389819604992);
* [J. Renée Beach warned about single pixel with interfering with screen readers vocalisation](https://medium.com/@jessebeach/beware-smushed-off-screen-accessible-text-5952a4c2cbfe#.vcd5xlpgg) solved with `white-space`.

See [the detailed post on Hugo Giraudel's blog](http://hugogiraudel.com/2016/10/13/css-hide-and-seek/).

Also kinda related to issue #20732 :)

Please let me know if you find any trouble with this technique. Thanks a lot!

* Reorder declarations

To pass Hound.
Didn't think `clip` and `clip-path` would be considered as unrelated…

* Trailing spaces

* Last trailing space?

* Remove margin: -1px

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