Not explicitly stated that content indentation level of 0 is prohibited

[terry422]

The following YAML document is invalid because the content line of the block scalar does not have any indentation:

|
scalar

I believe this is intended, as c-indentation-indicator omits 0. However, I could not find a clause that explicitly precludes a non-empty line with zero indentation, when the indicator is omitted and the indentation level is auto-detected.

[ingydotnet]

The YAML is actually valid according to the spec, but about half the parsers we test find it invalid.

https://play.yaml.io/main/parser?input=fApmb28K

Here's another interesting case where all known parsers interpret different than the spec:

https://play.yaml.io/main/parser?input=fDIKICBmb28K

It all has to do with the (flawed imho) concept of indentation starting at -1.

A better approach might be to define indentation as a state of indented or not-indented.