CSS hack writing distinguishes different browsers IE6 IE7 Firefox:

Difference IE6 with FF:
Background:orange;*background:blue;

Distinction between IE6 and IE7:
Background:green!important;background:blue;

Difference IE7 with FF:
Background:orange; *background:green;

Difference ff,ie7,ie6:
Background:orange;*background:green!important;*background:blue;

Note: IE can recognize *; standard browsers (such as FF) cannot recognize *;
IE6 can recognize *, but cannot recognize!important,
IE7 can identify the *, also can identify!important;
FF does not recognize *, but can identify!important;

IE6 IE7 FF
*√√x
!importantx√√

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Add: IE6 supports underscores, IE7 and Firefox do not support underscores.

So we can distinguish Ie6,ie7,firefox
: Background:orange;*background:green;_background:blue;

Note: No matter what method, the order of writing is Firefox written in the front, IE7 written in the middle, IE6 written in the last.