From IndieWeb

Fatwigoo, or FOUSVG, is an embedded SVG image that depends on (external) CSS for its sizing. These images are blown up to fill the entire screen if the CSS fails to load.

SVG elements embedded in HTML with the svg element default to a width of the entire browser viewport, scaling its height to that size. If a website depends on CSS to resize the SVG and this does not load for any reason (e.g. bad network or blocking extension) the user will have their screen filled with just the one icon. Similarly, external CSS will mostly not be available when content is included on other pages, e.g. a reply-context, planet or comment (but this applies to other images too).

An example on the IndieWeb, Eddie Hinkleโ€™s home page on 2018-03-01 without CSS fills a big part of the viewport with its menu icon:

See Also