What HTTP header indicates when Index.html has been served?

What HTTP header indicates when Index.html has been served?

Excuse the awkward title: I'm building a simple web server (don't ask...)
and have this problem:
The browser requests mydomain.com/MyFolder
My server spots this is a folder, so instead, delivers
mydomain.com/MyFolder/index.html
All fine so far, except that index.html has link to mycss.css, but the
browser requests it as a top-level file mydomain.com/mycss.css instead of
mydomain.com/myFolder/mycss.css.
Is there some HTTP header that needs setting up to indicate that a
different page has been served? I've tried returning Content-Location:
/myFolder/index.html, but without any visible success.
index.html basically contains this:
<link rel="stylesheet" href="mycss.css" />