This is rather similar to bug 126646. We produce an off by one pixel rendering when subpixel is on, though flooring it to CSS pixel would defeat the purpose of subpixel positioning the column content. m_columnWidth = std::max<LayoutUnit>(0, (availWidth - ((m_columnCount - 1) * colGap)) / m_columnCount); failed cases: fast/multicol/newmulticol/column-rules-fixed-height.html fast/multicol/newmulticol/columns-shorthand-parsing.html fast/multicol/newmulticol/layers-in-multicol.html
Talk to Hyatt.
Created attachment 221301 [details] rendering snapshots
<rdar://problem/15827900>
FF(v26) rendering is in-between subpixel off and subpixel on. Prefer current subpixel rendering.