New observation target proposal

[atotic]

Original proposal was to observe target's clientHeight. This was inadequate, as explained in #4.
Here is the new proposal:

1. Observe the target's content size
   • Because authors manipulate DOM with respect to content size. Content size is well-specified in CSS. "content size like" properties have various quirks, lets use content size directly.
2. Report observations with content rectangle, whose:
   • size is content size,
   • position top is padding-top, left is padding-left

Having content rect position be padding-top/left is useful for absolute positioning of target's children. Absolute position coordinate space origin is topleft of the padding rect.

The new ResizeObserverEntry IDL would be:

interface ResizeObserverEntry {
    readonly attribute Element target;
    readonly attribute DOMRectReadOnly contentRect;
};

What makes me happy about content rect entry is that all the information needed to position/show/hide children is right there.

[atotic]

Spec has been updated with observing content size. goodbye clientWidth....