Non-standard: This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.
msFirstPaint is a read-only property which gets the time when the document loaded by the window object began to be displayed to the user.
Put another way,
This proprietary property is specific to Internet Explorer and Microsoft Edge.