Unstyled pages. Not all of them are FOUCs. Some happen while deploying due to temporary 404s of images, CSS, or JS.
UI Drafter avoids those issues by nesting assets in subdirectories:
…and deploying them in two steps:
# Subdirectories first rsync -r --include '*/' --exclude '/*' $SRC/ $DEST/ # Then the top level files (HTML documents) rsync --times $SRC/* $DEST