Builds a CSS timing function from the two control points or one of 29 presets (CSS keywords, Penner sine/quad/cubic/quart/expo/circ/back families, Material curves). The curve is solved numerically to tabulate progress over time, matched to the closest preset, and approximated as a CSS linear() function for engines that prefer it. The preview draws the curve with its handles and animates three lanes — custom, linear and an opacity/scale demo — with pure CSS. Everything runs locally.