A YAML file use spaces as indentation, you can use 2 or 4 spaces for indentation, but no tab. In other words, tab indentation is forbidden:
Why does YAML forbid tabs?
Tabs have been outlawed since they are treated differently by different editors and tools. And since indentation is so critical to proper interpretation of YAML, this issue is just too tricky to even attempt. Indeed Guido van Rossum of Python has acknowledged that allowing TABs in Python source is a headache for many people and that were he to design Python again, he would forbid them.
(source: YAML FAQ (thanks to Destiny Architect for the link))
For example, the Symfony configuration file can be written with 2 or 4 spaces as indentation:
4 spaces
doctrine:
dbal:
default_connection: default
2 spaces
doctrine:
dbal:
default_connection: default
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