In an effort to avoid tying block parsing to the DOM, we should not expect block implementers to provide custom matchers for attribute parsing.

Examples:

• https://github.com/WordPress/gutenberg/blob/29462aa/blocks/library/image/index.js#L35
• https://github.com/WordPress/gutenberg/blob/29462aa/blocks/library/quote/index.js#L18-L30

Instead, we should decide between:

• Expecting these attributes to be serialized in the block comment
• Providing additional matchers to satisfy the need required by these blocks

To discourage taking advantage of matchers currently receiving the DOM node, we could consider adding an internal flag, and picking only keys from the block attributes property whose values are an allowable matcher.