What is the 'self' of sandboxed page?

[shhnjk]

Following test shows that all browsers treat 'self' as url's origin for sandboxed page.

https://test.shhnjk.com/csp_sandbox.php?xss=%3Cscript%20src=/alert.js%3E%3C/script%3E%3Cimg%20src=/test.jpeg%3E&csp=allow-scripts%20allow-modals;%20default-src%20%27self%27

But the spec doesn't seem to say so.

What is the right behavior here?