Have mach update (not just init) submodules as needed. Fix #3447
Merge pull request #3448 from SimonSapin/mach-update-submodules

Have mach update (not just init) submodules as needed. Fix #3447
Add support for min-width and max-width to images.

This fixes the images on the homepage of github.
Merge pull request #3449 from glennw/img-min-max

Add support for min-width and max-width to images.
Update web-platform-tests.
Merge pull request #3446 from Ms2ger/update-wpt

Update web-platform-tests; r=Manishearth
Add support for font-size relative size keywords. Fix #3423
Merge pull request #3453 from SimonSapin/font-size-relative-keywords

Add support for font-size relative size keywords. Fix #3423