[Bug] Proposed CC0 1.1 contains invalid encoding declaration

## Description

According to [the HTML Standard](https://html.spec.whatwg.org/multipage/), a [`meta`](https://html.spec.whatwg.org/multipage/semantics.html#meta) element can have its [`http-equiv`](https://html.spec.whatwg.org/multipage/semantics.html#attr-meta-http-equiv) attribute set to [`content-type`](https://html.spec.whatwg.org/multipage/semantics.html#attr-meta-http-equiv-keyword-content-type). In this situation, the `http-equiv` attribute is in [the Encoding declaration state](https://html.spec.whatwg.org/multipage/semantics.html#attr-meta-http-equiv-content-type). [The proposed CC0 1.1](https://github.com/creativecommons/creativecommons.org/blob/cc0-1-1-proposed/docroot/legalcode/zero_1.1.html) has an `http-equiv` attribute that’s in the Encoding declaration state.
```xhtml

```
According to the HTML Standard, “The [Encoding declaration state](https://html.spec.whatwg.org/multipage/semantics.html#attr-meta-http-equiv-content-type) may be used in [HTML documents](https://dom.spec.whatwg.org/#html-document), but elements with an [`http-equiv`](https://html.spec.whatwg.org/multipage/semantics.html#attr-meta-http-equiv) attribute in that state must not be used in [XML documents](https://dom.spec.whatwg.org/#xml-document).” The proposed CC0 1.1 is an XML document.

## Reproduction

1. Get a copy of [the proposed CC0 1.1](https://github.com/creativecommons/creativecommons.org/blob/cc0-1-1-proposed/docroot/legalcode/zero_1.1.html).
2. Change its file extension to `.xht` or `.xhtml`.
3. [Try validating it.](https://validator.w3.org/nu/)

## Expectation

The page validates successfully.

## Resolution

- [x] I would be interested in resolving this bug.