That sort of works. However, if there's a page that has a partial block on it, like an ad or something that is NOT in an iframe (which almost none are anymore), the div and body styles gets applied to the entire page.
Here's a modification I made that definitely works for everything. And I made a dashed border instead, but you can turn it back to solid. I also changed the scroll feature of the div so when using the filter for advertisements, the block alert can be scrolled.
<style>*{margin:0 0 8}body{background:#eee;}#b{padding:4%;font:0.9em/1.3em arial;height:100%;overflow:auto}#d{border:3px dashed #ccc;padding:4%;background:#fff}i{display:block;padding:1%;kground:#fdd6d3}h3{color:#753;}</style><div id=b><div id=d><h3>Page Blocked</h3><p><b>COMPANY's</b> security policy prohibits access to:</p><i>$URL_CATEGORY</i><p>Repeated attempts to access blocked content may be investigated.</p></div>
That's 422 characters. Slightly less than the 427 (also used COMPANY instead of <Institution Name>) example above however much more functional and less disruptive of contents of the original page.