The implementation of an extended API of text metrics started in 2014: https://groups.google.com/a/chromium.org/d/msg/blink-dev/SgofW_bQ3ps/jNYamQHplAMJ But was never shipped, and had multiple undefined values that were not addressed for years. The spec has now evolved and we intend to ship actualBoundingBoxLeft, actualBoundingBoxRight, actualBoundingBoxAscent and actualBoundingBoxDescent according to the current spec: https://html.spec.whatwg.org/multipage/canvas.html#textmetrics