getClientRects Measures Text in Subpixels, and the Subpixels Name the Renderer

getClientRects and canvas measureText return the sub-pixel width of rendered text. At a fractional font size that width comes out different on FreeType than on DirectWrite or CoreText, because the three scalers round the glyph advance on different grids. One measurement at font-size:24.5555px separates a Linux text stack from a Windows or macOS one, and a custom Chromium build on Linux carries the Linux value no matter what OS it claims.