If a User Agent(UA) does not support a particular value, it will ignore that value when parsing
style sheets, as if that value was an illegal value . For example:
Example(s):
h3 { display: inline; display: run-in; }
A UA that supports the ’run-in’ value for the ’display’ property will accept the first
display declaration and then “write over” that value with the second display
declaration. A UA that does not support the ’run-in’ value will process the first display
declaration and ignore the second display declaration
So always remember when you’re writing some CSS always use one generic and one specific value for the property. Only in the cases when you’ve to create browser specific code.
Mostly avoid this act. 