To ensure compatibility with IE 6, you can use the following code:
filter: alpha(opacity=70);
It is important to note that supporting IE 6 should only be considered if absolutely necessary.
Update
As suggested by Joeytje50, you may also want to include the -ms-filter
attribute:
-ms-filter: "progid:DXImageTransform.Microsoft.Alpha(Opacity=70)";
The filter
attribute applies to all versions of Internet Explorer, while -ms-filter
is specifically for IE8 and newer versions.
According to msdn.microsoft.com:
The -ms-filter attribute is an extension to CSS, serving as an equivalent to filter in IE8 Standards mode. When using -ms-filter, remember to enclose the progid in single or double quotes and separate multiple values with commas.