2.2.27 [W3C-HTML52] Section 22.214.171.124. Serializing bitmaps to a file
C0045: Many image formats other than PNG are supported
The specification states:
... Serializing bitmaps to a file ... User agents must support PNG ("image/png"). User agents may support other types. If the user agent does not support the requested type, it must create the file using the PNG format.
Many image formats other than PNG are supported (gif, jpeg, ico, bmp, etc.).