Actually, transform is in the CSS3 working spec[0]. Until the standard reaches the "Candidate Recommendation" stage, the W3C encourages browser vendors to use prefixes (-webkit, -moz, etc).
DXImageTransform.Microsoft.Alpha, on the other hand, wouldn't be part of CSS3, and uses its own specific syntax.
Is there any technical reason why using webkit's browser specific css attribute and firefox's browser specific css attribute is any better than using IE's browser specific css attribute?
`filter:` syntax isn't compatible with CSS tokenizer (IE<8 is unparseable without special tokenizer state set by the parser, IE8 uses non-CSS syntax in CSS string).
IE's transform filter uses only matrix, which scares the hell out of designers.
DXImageTransform.Microsoft.Alpha, on the other hand, wouldn't be part of CSS3, and uses its own specific syntax.
[0]: http://www.w3.org/TR/css3-3d-transforms/#transform-property