<body>This line is pushed to left by 1px. (body's padding-left is 0.5em with font-size: 99%.)</body>
Created attachment 219279 [details]
This is the result of clamping float values at LayoutUnit(float) to int when minimumValueForLength() is called.
computed minimum value: 6.8;
subpixel on: LayoutUnit value -> 435 (which eventually gets pixelsnapped to 7px)
subpixel off: LayoutUnit value -> 6 as the result of the clamping.
It mainly (most visibly) effects paddings and margins, but it can push pixels off by 1 at other places too (like with text-indent, width properties)
I think it's OK to have a behavior change with subpixel when authors are using percentages. We should check Firefox behavior.
Created attachment 219704 [details]
Rounding these values would also regress some content including wikipedia.
Created attachment 219705 [details]
regressed test case (wikipedia)
Created attachment 221209 [details]
Testing FF's rendering.
The new off-by-one rendering matches FF's (v26.0) rendering. (top to bottom: subpixel on, FF, subpixel off)