visible stuff
depending on how much you wanna format, the 'visible stuff' can barely be formatted from within. if you "[lmg]urlblabla[/img]" after = you can put an image in.
If you only need heavy formatting on either the hidden or visible side, then you can see if collapse or collapse2 suffice. If you want formatting on both sides, that are the same, like both italic and colored the same, you can use these functions outside the collapse and it will use them too.
If they are completely different styles you have to get creative like formatting outside and reformatting inside.
Here an example of a simple yet halfway elegant formatting