<style>html { font-size: 15px; }</style> alters width calculations

Created by: SuzanneSoy

In https://jsfiddle.net/xvc1e6qL/, $x\vcenter{aaaaaaaaaaaaaaaaaaaaaaaaaaa\\bbbbbbb}y$ renders with extra space on the right when the page's stylesheet sets html { font-size: 15px; } (on Chromium 57.0.2987.110 (64-bit) if that matters). Here's a screenshot with HTML-CSS mode:

Using an array (with \! on each side, source code here: https://jsfiddle.net/k9vbs42e/) produces the same result.

The size of the error seems proportional to the number of as https://jsfiddle.net/wevnsjyn/, here's a screenshot with CommonHTML mode using array:

Different values for font-size produce different, seemingly random spacing issues:

15px: 14px: 13px: 12px: