Added some CSS questions

Added question re: z-index and stacking context
Typo fixed
Merge pull request #135 from tjwudi/master

Typo fixed
Merge pull request #134 from mattiasw/master

Translate new CSS questions that have been added to the English version.
Merge pull request #133 from NkS90/master

Added some CSS questions
fixed typo

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>
translate some css question.

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>
fixed error title.

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>
Merge pull request #136 from appleboy/patch

Some patch for Chinese Traditional
Update Swedish translation to the latest English version