The YAML spec clearly states:
Mappings use a colon and space (“: ”) to mark each key: value pair.
So this is legal:
foo: bar
But this, Ain't:
foo:bar
I see many people online that are ranting about the space. I think they have a point. I got burned by it several times myself.
Why is the space mandatory? What was the design consideration behind it?
It's easy to miss, because that specification uses the bizarre convention of only highlighting the last character of an internal link, but the “: ” in the section you quote is actually a link to another section of the specification which answers your question:
Normally, YAML insists the “:” mapping value indicator be separated from the value by white space. A benefit of this restriction is that the “:” character can be used inside plain scalars, as long as it is not followed by white space. This allows for unquoted URLs and timestamps. It is also a potential source for confusion as “a:1” is a plain scalar and not a key: value pair.
So the motivation is that you can write lists such as this without requiring any quoting:
useful_values:
- 2:30
- http://example.com
- localhost:8080
If the space was optional, this could end up being ambiguous, and interpreted as a set of key-value pairs.
Aside: Here's a snippet of JS to make the link formatting on that document less useless.
document.styleSheets[0].insertRule('a[href^="#"] { color: #00A !important; text-decoration: underline !important; background: none !important; }', 0);
Update: The revised "1.2.2" specification, published in 2021, uses more conventional link formatting, but that particular hyperlink is no longer present. The wording quoted is unchanged, however.