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