float:right or float:start for rtl"The left outer edge of a left-floating box may not be to the left of the left edge of its containing block. An analogous rule holds for right-floating elements."
"Each box is given a position with respect to its containing block, but it is not confined by this containing block; it may overflow." Note the wording "may overflow." In this test case, the overflow is hidden outside the left edge of the <html> (viewport).
"The containing block in which the root element lives is a rectangle called the initial containing block. For continuous media, it has the dimensions of the viewport and is anchored at the canvas origin; it is the page area for paged media. The 'direction' property of the initial containing block is the same as for the root element"
"This property specifies whether content of a block-level element is clipped when it overflows the element's box. It affects the clipping of all of the element's content except any descendant elements (and their respective content and descendants) whose containing block is the viewport or an ancestor of the element."
"Even if 'overflow' is set to 'visible', content may be clipped to a UA's document window by the native operating environment."
"This property specifies the base writing direction of blocks and the direction of embeddings and overrides (see 'unicode-bidi') for the Unicode bidirectional algorithm. In addition, it specifies the direction of table column layout, the direction of horizontal overflow, and the position of an incomplete last line in a block in case of 'text-align: justify'."