Comment by AdieuToLogic

A slightly simpler version of same is:

  for PAGE in *.page
  do
    cat header.html "$PAGE" footer.html > "$PAGE.html"
  done

As noted in a peer comment, the cat[0] command supports concatenating multiple files to stdout in one invocation.

HTH

EDIT: if you don't want the output file to be named "a.page.html" and instead it to be "a.html", replace the above cat invocation with:

  cat header.html "$PAGE" footer.html > "${PAGE%.page}.html"

This syntax assumes use of a POSIX/bash/zsh shell.