If your game is under 5mb (since it seems you said loading a 2mb spritesheet loaded 70% of the game), maybe you don't need a loader layout at all. It's most appropriate for very large games only. These days the average web page size is something like 2-3mb, so if your game is in that ballpark, it will load as quickly as a normal web page.
my game is over 10mb, with code and sounds.
and even 5 mb can load a lot of time on the bad connection, in the countries where people playing web games, internet speed is usually low.
and forgot that I also have a progress bar that consists of bar background and bar itself.
And see that bar background was placed in the 3rd biggest atlas while bar in the 4th also biggest atlas. so, to load tiny pictures loader got to download 2 big atlases.
sometimes images are divided successfully, as separated images, or placing in the small atlas, but the fact that this important moment is a matter of luck is disturbing