<summary> not handled correctly — incorrect DTD?

[Wim Leers]

Steps to reproduce

Originally reported at ​https://www.drupal.org/node/2656566.

1. Disable ACF.

2.. Enter source mode and paste

<details>
	<summary>
		<div>summary</div>
	</summary>
	<div>content</div>
</details>

3. Exit source mode.
4. Enter source mode again.

Expected result

Markup is identical to the pasted markup.

Actual result

<details><summary></summary>

<div>summary</div>

<div>content</div>
</details>

Note how <summary> is empty.

Suspected cause: <summary> incorrectly defined in CKEditor's DTD?

Other details (browser, OS, CKEditor version, installed plugins)

Drupal 8 (8.0.3 or 8.1.x). Go to /node/add/article, choose the "Full HTML" text format (to disable ACF) and then apply the STR above.