My actual viewport width is pixels

All iframes render the same document, but the styling of certain elements is controlled by each element's responsive style attribute.

And Here's Why

Give editors the option to set a headline's actual font-size for a viewport depending on the headlines length

That's A Good Thing

It enables you to set styles based on content properties and arbitrary viewport dimensions, without polluting your stylesheets and sources with one-off classes for very specific cases.