Highest：html file
High：css file
MIddle：preload(as style == as script == as image)
Low：head resource
lowest：body file
XMLHttpRequest level == other file level
Image(In viewport) level > Image(Not in viewport) level
head script level > defer script level > body script level
defer script level == async script level
if we want loading image as soon as possible,we need use preload for this image and put this code in front of the rest of the preload