What Nick did

First he combined all svg into one single svg container

this saves the opening and closing svg tags (not much but it saves a little bandwidth)

Then he put this big svg element into every html document

this saves a little bit of bandwidth (no extra HTTP request), but it cost much more memory because the svgs are implemented for every site which is not efficient

Finally he removed the svgs out of the svg element which are not used for this site

this is memory efficient

In the html doc he used:

<svg>
    <use xlink:href="#twitter" />
</svg>

The svg element indicates that this is an svg. The use element looks for a svg that is already used with the specified class in a symbol element inside of a svg element;