The icons are drawn on a 16x16 grid. In order to avoid excessive sub-pixel dithering, the main vertical and horizontal lines are aligned on that grid. This necessarily means that some appear off-center, in some contexts. Where that creates a visible regression, I'd prefer we make adjustments in the CSS rather than the icon itself. This keeps the icons consistent with each other, and avoids "fixing" an alignment or spacing issue in one case, only to break it in others. This one (add.svg) is used in several other places in addition to the tab stripo.
Bug 1706695 Comment 3 Edit History
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The icons are drawn on a 16x16 grid. In order to avoid excessive sub-pixel dithering, the main vertical and horizontal lines are aligned on that grid. This necessarily means that some appear off-center, in some contexts. Where that creates a visible regression, I'd prefer we make adjustments in the CSS rather than the icon itself. This keeps the icons consistent with each other, and avoids "fixing" an alignment or spacing issue in one case, only to break it in others. This one (add.svg) is used in several other places in addition to the tab strip.