web-platform-tests have been running against GeckoView for a couple of months, and we've found that several tests in this component are unstable and need investigation.

Here is the state of disabled tests according to testing/web-platform/meta at revision 241af4dbb964

css/css-backgrounds/background-attachment-local/attachment-local-positioning-5.html
    ['  bug 1550895 (frequently fails on geckoview)']
css/css-backgrounds/background-size/background-size-cover-svg.html
    ['  bug 1550895 (frequently fails on geckoview)']
css/css-backgrounds/border-image-width-005.xht
    ['  bug 1550895 (frequently fails on geckoview)']
css/css-backgrounds/border-image-width-006.xht
    ['  https://bugzilla.mozilla.org/show_bug.cgi?id=1554777']
css/css-backgrounds/border-image-width-007.xht
    ['  bug 1550895 (frequently fails on geckoview)']
css/css-backgrounds/border-image-width-008.html
    ['  bug 1550895 (frequently fails on geckoview)']

Filing this bug to track the work needed to turn them all back on.

Note that there are many other intermittents in this directory that have not been disabled yet.