[filter-effects] hue-rotate() and saturate() filters

If this has already been discussed and resolved at some stage, I apologize
for the duplication + noise in advance.

Prompted by a stack-overflow question on css filters, I did an experiment
comparing the results of CSS hue-rotate() and saturate() filters with their
manual equivalent (explicitly changing hsl() colors).

Hue-rotate experiment here: http://codepen.io/mullany/pen/fxsuE
Saturate experiment here: http://codepen.io/mullany/pen/rpgHu

As you can see, the hue-rotate does a poor job of conserving saturation and
lightness, and the saturate does a poor job of conserving lightness. These
are extreme examples using fully saturated colors - less saturated and
lighter colors don't seem to diverge quite so much as these do.

I do understand that because these filters are a linear matrix
approximation and remain in RGB space, there will be inaccuracies when
trying to do HSL ops, but these seem very extreme to me, and very
unexpected. But they seem to be consistent between Firefox and Webkit/Blink
- (when converted to SVG equivalents) - so I do not think this is a bug.

Has there been any thought of converting content to HSL before applying
these filter shorthands? Or adding true HSL primitives to the filter
toolbox? (Something like an feFuncH or feFuncS).

-- michael