The spec has been updated so "normal" is stretch for regular grid items, but for replaced elements it's "start". It was discussed and agreed on the following CSS WG issue: https://github.com/w3c/csswg-drafts/issues/523 We should implement this to pass the following test from the W3C suite: css-grid-1/grid-items/grid-items-sizing-alignment-001.html
This has been fixed already in r213480 (bug #169203). That patch was importing: https://chromium.googlesource.com/chromium/src/+/fad39c71033eea490d8437ca186529c8e87125c2