"On the other hand, once you've instantiated a custom element, its qualities—including its shadow DOM subtree—are inherent part of element's identity.", is the last line of section 7. The last part, after the dash, is missing an 'an.'

It should read:

"On the other hand, once you've instantiated a custom element, its qualities—including its shadow DOM subtree—are *an* inherent part of element's identity." with asterisks marking the change.