some of the example templates fail csslint

Created by: cvrebert

Upon trying to add the examples' CSS to the csslint Grunt task, I discovered that some of them fail csslint:

Running "csslint:src" (csslint) task
Linting docs/examples/blog/blog.css ...ERROR
[L44:C1]
Rule is empty. Rules without any properties specified should be removed. (empty-rules)
[L73:C3]
vertical-align can't be used with display: block. Certain properties shouldn't be used with certain display property values. (display-property-grouping)
Linting docs/examples/grid/grid.css ...ERROR
[L21:C3]
Duplicate property 'background-color' found. Duplicate properties must appear one after the other. (duplicate-properties)
[L22:C3]
Duplicate property 'border' found. Duplicate properties must appear one after the other. (duplicate-properties)
Linting docs/examples/justified-nav/justified-nav.css ...ERROR
[L44:C3]
Duplicate property 'background-image' found. Duplicate properties must appear one after the other. (duplicate-properties)
Linting docs/examples/non-responsive/non-responsive.css ...ERROR
[L41:C3]
Duplicate property 'background-color' found. Duplicate properties must appear one after the other. (duplicate-properties)
[L42:C3]
Duplicate property 'border' found. Duplicate properties must appear one after the other. (duplicate-properties)