z-index doesn't explain -1

[ianschmitz]

I noticed when reading the z-index article, that you show in an example "z-index: -1" but don't explain it's usage.

[SaraSoueidan]

I must have assumed that by that point the value of z-index was self-explanatory, but you are right: unless I mention the stack in this example, I should add some explanation to it. Thank you for the heads up!