[css-align] Centering in the viewport

Dear All,

when investigating modern ways of centering the viewport I found these
examples:
https://www.w3.org/Style/Examples/007/center

Especially the last one utilizing 'transform' property looks promising:
https://www.w3.org/Style/Examples/007/center-example2

However, in all major browsers, when the window height is small so
scrollbars are activated, the top of the box is cropped and it cannot be
scrolled any more.

Is such rendering incorrect or there is something wrong in the style
definition?

Thanks,

Jan