Commit 519af3 deprecated one color stops. However some test cases use that, which causes warnings that result in test errors:

/build/gtk+3.0-3.21.4/testsuite/reftests/background-image-multiple.ui: 
(lt-gtk-reftest:26732): Gtk-WARNING **: Theme parsing error: background-image-multiple.css:4:54: Using one color stop with linear-gradient() is deprecated.
FAIL

  /build/gtk+3.0-3.21.4/testsuite/reftests/background-size-zero.ui:    
(lt-gtk-reftest:26748): Gtk-WARNING **: Theme parsing error: background-size-zero.css:4:50: Using one color stop with linear-gradient() is deprecated.
FAIL

/build/gtk+3.0-3.21.4/testsuite/reftests/linear-gradient-transition-to-other.ui: 
(lt-gtk-reftest:26791): Gtk-WARNING **: Theme parsing error: linear-gradient-transition-to-other.css:6:58: Using one color stop with linear-gradient() is deprecated.
FAIL

There are other css test suite errors:

  /build/gtk+3.0-3.21.4/testsuite/css/style/colornames.ui:             **
ERROR:/build/gtk+3.0-3.21.4/./testsuite/css/style/test-css-style.c:115:load_ui_file: assertion failed (error == NULL): colornames.css:120:42'rebeccapurple' is not a valid color name (gtk-css-provider-error-quark, 1)
FAIL

  /build/gtk+3.0-3.21.4/testsuite/reftests/animation-direction.ui:     
(lt-gtk-reftest:26677): Gtk-WARNING **: Theme parsing error: animation-direction.css:25:32: Junk at end of value for animation-direction
FAIL

/build/gtk+3.0-3.21.4/testsuite/reftests/animation-fill-mode-iteration-count.ui: 
(lt-gtk-reftest:26716): Gtk-WARNING **: Theme parsing error: animation-fill-mode-iteration-count.css:38:32: Junk at end of value for animation-direction
FAIL

  /build/gtk+3.0-3.21.4/testsuite/reftests/label-sizing.ui:            
(lt-gtk-reftest:26766): Gtk-WARNING **: Theme parsing error: label-sizing.css:2:8: not a number
FAIL