E307 - Iframe border_width or shadow_size must be >= 0

Cause

An iframe element has a negative border_width: or shadow_size: value. Both are pixel measurements and must be non-negative.

Example

{ iframe_html: "<p>hi</p>", position: [0, 0], width: 50, height: 50,
  border_width: -1 }

How to fix

Use 0 (the default — no border / no shadow) or a positive pixel value.