The filter option on the style attribute in HTML

Alpha blending, cast shadows and other effects on HTML elements such as tables, tablecells and images. (IE only)

Alphablending example:

HTML:
<IMG style="FILTER: Alpha(opacity=50)" src=http://www.funkelab.com/files/chemicals.jpg>

X-Ray example:

HTML:
<IMG style="FILTER: Xray" src=http://www.funkelab.com/files/chemicals.jpg>