Ok, here's the compromise language... on subsequent calls to getContext()
for the same context, attrs must be either unspecified (in which case you
get what's there), or if given, must be compatible with what's already
active (or you get a hard error).
object getContext(in DOMString contextId, in optional any attributes)
A canvas may be rendered to using one or more contexts, each named by a
string context ID. For each canvas, there is a set of zero or more active
contexts. The getContext() method is used to obtain a particular rendering
context for the canvas.
'contextId' must be a string naming a canvas rendering context to be
returned. For example, this specification defines the '2d' context, which,
if requested, will return either a reference to an object implementing
CanvasRenderingContext2D or null, if a 2D context cannot be created at this
time. Other specifications may define their own contexts, which would
return different objects.
If getContext() is called with a context ID that the implementation does not
support, it must return null.
For supported context IDs, the getContext() method behaves differently
depending on whether the requested context ID is already an active context
for the canvas.
If there are no active contexts for the canvas, the implementation should
attempt to create the specified context for the canvas.
If there are one or more active contexts and a context ID that is not
currently active is requested, it is up to the implementation to determine
whether the requested context can be used simultaneously with all currently
active canvas contexts. If simultaneous rendering with the requested
context is not possible, getContext() must return null. Otherwise the
implementation should attempt to create the specified context for the
If a new context is to be created, the optional 'attributes' parameter must
be either unspecified or an object specific to the context being requested.
An unspecified value indicates a default set of attributes, as defined by
the context. Unknown attributes should be ignored by the context.
If a new context is successfully created, a reference to an object
implementing the context API is returned and the new context is added to the
list of active contexts for the canvas.
If a context ID that is already an active context for the canvas is
requested, then if the optional 'attributes' parameter is unspecified, a
reference to the existing context object must be returned. If the
'attributes' parameter is specified, the context must determine whether the
given set of attributes is compatible with the attributes set when the
context was created. If the attributes are compatible, the existing context
object must be returned. If the attributes are not compatible, getContext()
must raise an INVALID_STATE_ERR exception.
If multiple rendering contexts are active, they all render to the same
canvas bitmap; they are not layered or otherwise isolated. Changes made to
the canvas bitmap with one context must be immediately visible to any other
active contexts on the canvas. It is up to the implementation to manage
synchronization issues associated with rendering with different contexts to
the same canvas.