Merge pull request #400 from Handfeger/master

Using square brackets instead of curly brackets to access string content
Fix typo in docs/docs.md

constrast -> contrast
Merge pull request #406 from TimWolla/patch-1

Fix typo in docs/docs.md
Adding viewport directives
Adding to directive tests
Merge pull request #407 from AnhNhan/master

Adding `@viewport` directive and relatives