The specification in a few placed contains something like this:

"
Let visible width be element’s bounding rectangle’s (max(x coordinate, x coordinate + width dimension) – the current browsing context’s innerWidth).
"

E.g. here:
https://www.w3.org/TR/webdriver/#element-interactability

Let's suppose we have rectangle with x = 1, and width = 6,
and viewPort innerWidth is 10.

According to this formula:

visible_width = max(1, 1 + 6) - 10 = -3 (MINUS 3).

And then we have x center point at 1 - 3/2 = -0.5 (MINUS 0.5 which is out of the element boundaries).

Maybe there should be such a wording:
"
Let visible width be element’s bounding rectangle’s (min(max(x coordinate, x coordinate + width dimension), the current browsing context’s innerWidth)).
"

================

Also there are everywhere mess between visible width and height, like this:

"
Let vertical centre offset be (visible width / 2.0). // height and not width must be here

Let horizontal centre offset be (visible height / 2.0). // width and not height must be here.

Let centre point be a pair of rect’s (rectangle x coordinate + horizontal centre offset) and (rectangle y coordinate + vertical centre offset)
"

================

Also there is copy/paste bugs. Like: "
Let visible width be element’s bounding rectangle’s (max(x coordinate, x coordinate + width dimension) – the current browsing context’s innerWidth).

Let visible height be element’s bounding rectangle’s (max(y coordinate, y coordinate + height dimension) – the current browsing context’s innerWidth).
"

The second sentence contains innerWidth again instead of innerHeight.