other block elements include the body element
their clientWidth attributes are all equal to the size of the padding-box area
their offsetWidth attributes are equal to the size of the border-box area
but for the html element, the value of
docuement.documentElement.clientWidth has nothing to do with the area size of the html element
it just has something to do with whether the scroll bar appears, the size of the scroll bar is smaller
the value of docuement.documentElement.offsetWidth is the same as the border-box of the html element
that is to say, docuement.documentElement.clientWidth is very special relative to other elements and attributes
Why should browsers treat this attribute of html element differently?