[css-contain] The interaction between containment on the HTML element, and overflow propagation is insufficiently specified

[smfr]

The note at https://www.w3.org/TR/css-overflow-3/#overflow-propagation says "Using containment disables this special handling of the HTML body element."

However, it doesn't say which element containment is being applied to in this scenario; it might be the body or the html.

So the behavior of this scenario isn't very clear:

html { contain: layout; }
body { overflow: hidden; }

Does overflow:hidden propagation from the body to the viewport, or does containment on the html prevent that propagation?

Test case at https://jsfiddle.net/qj9v0h2t/

[frivoal]

It needs republishing, but css-contain-1 gives details at https://drafts.csswg.org/css-contain-1/#c3

Additionally, when the used value of the contain property on either the HTML html or body elements is anything other than none, propagation of properties from the body element to the initial containing block, the viewport, or the canvas background, is disabled. Notably, this affects:

• writing-mode, direction, and text-orientation (see CSS Writing Modes 3 § 8 The Principal Writing Mode)

• overflow and its longhands (see CSS Overflow 3 § 3.5 Overflow Viewport Propagation)

• background and its longhands (see CSS Backgrounds 3 § 2.11.2 The Canvas Background and the HTML Element)

Note: Propagation to the initial containing block, the viewport, or the canvas background, of properties set on the html element itself is unaffected.

css-overflow-3, on the other hand, could be clearer.