Field Block.contentSource regenerates the block's original HTML content. It accomplishes this by calling PHP function serialize_block in createBlock.

However, this value will not be exactly the same as the one stored in the DB (i.e. the one generated via JavaScript/Gutenberg). This is because:

• the default attributes (eg: an empty array) should not be included, however serialize_block does not remove them
• attributes stored inside the innerHTML (eg: the HTML for the paragraph block) are also stored within the attributes (and thus this string is duplicated!)

A better solution would be to retrieve the HTML content as is, already when parsing the blocks in class
WP_Block_Parser_Block (see wp-includes/class-wp-block-parser.php). But this is currently not supported!

TODO: If WP_Block_Parser_Block ever retrieves the original HTML source, then use this logic to resolve Block.contentSource.