[jlreq] Optimize the images (#218)

xfq has just created a new issue for https://github.com/w3c/jlreq:

== Optimize the images ==
It often takes tens of seconds to open jlreq, so IMHO it would be useful to improve the performance by:

* Changing the image format to optimized [WebP](https://caniuse.com/#feat=webp) or SVG (using the [picture](https://html.spec.whatwg.org/multipage/embedded-content.html#the-picture-element) element if needed), and/or
* Adding the [loading](https://html.spec.whatwg.org/multipage/embedded-content.html#attr-img-loading) attribute to the `img` elements

since over 90% of the bytes downloaded are images.

Please view or discuss this issue at https://github.com/w3c/jlreq/issues/218 using your GitHub account