Enumerates the unit types that an overlay viewport value may be.
The value is specified in CSS pixels.
The value is specified as a proportion of the viewing canvas. If the canvas is resized, the viewports dimensions will be recalculated accordingly. In this case the value should be specified in a range of 0.0 to 1.0.
The value is specified as a proportion its corresponding parameters calculated value. For example, given a viewport specified with width of 0.25 and unit type of ProportionOfCanvas, specifying height with value of 1.0 and type of ProportionOfOtherDimension will ensure that the calculated height value will be equal to the calculated width value should the canvas resize. In this case the value should be specified in a range of 0.0 to 1.0.