CSS needs cross-browser work

In IE10, the default CSS does not make it look like in other browsers, the window is either the same width as the entire window, or it is left-aligned when it's centered in other browsers.

Start by reproducing the IE10 layout and comparing it with the other browsers described in the issue. Inspect the polyfill's default CSS and the code that controls the dialog's width and centering. Done means the dialog matches the expected centered layout in IE10 without regressing other browsers.

Written by the indexing model from the issue text.