Three things:

1) You can combine the browser fallback if-else into a single line of code:
`(window.innerHeight || document.documentElement.clientHeight) - 40 + 'px'`

2) The `window.onload` function is unnecessary because this script is loaded in at the bottom of the `<body>` tag and is automatically run.

3) The `myHeight` variable is also unnecessary. The code that gets the viewport height could be put straight into `primaryHeader.style.height`. Perhaps even the `primaryHeader` variable is also not required.