An estimate of what fraction of the current navigation has been loaded.
Mac Catalyst 13.0+
This value ranges from 0.0 to 1.0 based on the total number of bytes expected to be received, including the main document and all of its potential subresources. After navigation loading completes, the estimatedProgress remains at 1.0 until a new navigation starts, at which point the estimatedProgress is reset to 0.0. The WKWebView class is key-value observing (KVO) compliant for this property.