What's a valid HTML5 document?
The title element is indeed required, but as Jukka Korpela notes, it also must be non-empty. Furthermore, the content model of the title element is:
Text that is not inter-element whitespace.
Therefore, having just a space character in the title element is not considered valid HTML. You can check this in W3C validator.
So, an example of a minimal and valid HTML5 document is the following:
<!doctype html><title>a</title>
This is the minimal HTML5-valid document:
<!doctype html><title> </title>
W3C HTML validator maintainer here. FYI with regard to the validator behavior, as of today, the validator now enforces the requirement in the HTML spec that the title element must contain at least one non-whitespace character -
http://validator.w3.org/nu/?doc=data%3Atext%2Fhtml%3Bcharset%3Dutf-8%2C%3C%2521doctype%2520html%3E%3Ctitle%3E%2520%2520%2520%3C%252Ftitle%3E