Microsoft has long supported its own way of creating many visual effects that are now featured in CSS, ever since IE 5.5. The problem is that the filters are rather long, complex, proprietary (supported only in IE) and somewhat difficult to understand; I find it easiest to use various tools to convert CSS into MS-specific filters, and then call those generated values with conditional comments.
Gradients are only supported in IE10+, but simple gradients (one color start, one color end, no stops) can be made by using
Opacity effects are relatively easy to replicate in IE8. You don’t even need to use a generator; just cut and paste the code below and change the value as needed (the Microsoft version of the filter measures opacity between 0 and 100, rather than 0 and 1):