All: Fix invalid URLs in sitemap

https://github.com/jquery/jquerymobile.com/actions/runs/6490030580/job/17625221359

```
DEBUG:scrapy.core.engine:Crawled (200) <GET https://api.jquerymobile.com/wp-sitemap.xml> (referer: None)
ERROR:scrapy.core.scraper:Spider error processing <GET https://api.jquerymobile.com/wp-sitemap.xml> (referer: None)
Traceback (most recent call last):
  …
  File "/home/seleuser/.local/share/virtualenvs/seleuser-AdYDHarm/lib/python3.10/site-packages/scrapy/spiders/sitemap.py"
  in _parse_sitemap
  File "/home/seleuser/.local/share/virtualenvs/seleuser-AdYDHarm/lib/python3.10/site-packages/scrapy/http/request/__init__.py"
  in self._set_url(url)
  File "/home/seleuser/.local/share/virtualenvs/seleuser-AdYDHarm/lib/python3.10/site-packages/scrapy/http/request/__init__.py"
  in _set_url
ValueError: Missing scheme in request url: //api.jquerymobile.com/wp-sitemap-posts-post-1.xml
2023-10-12 01:21:37 [scrapy.core.scraper] ERROR: Spider error processing <GET https://api.jquerymobile.com/wp-sitemap.xml> (referer: None)
```

Ref jquery/infrastructure-puppet#33

Author: Krinkle

CONTRIBUTING.md

@@ -1,5 +1,22 @@
-Welcome! Thanks for your interest in contributing to jquery-wp-content. You're **almost** in the right place. More information on how to contribute to this and all other jQuery Foundation projects is over at [contribute.jquery.org](https://contribute.jquery.org). You'll definitely want to take a look at the articles on contributing [to our websites](https://contribute.jquery.org/web-sites/) and [code](https://contribute.jquery.org/code).
+# Contributing
+
+Welcome! Thanks for your interest in contributing to jquery-wp-content. More information on how to contribute to this and other projects is over at [contribute.jquery.org](https://contribute.jquery.org). You'll definitely want to take a look at the articles on contributing [to our websites](https://contribute.jquery.org/web-sites/) and [code](https://contribute.jquery.org/code).
 
 You may also want to take a look at our [commit & pull request guide](https://contribute.jquery.org/commits-and-pull-requests/) and [style guides](https://contribute.jquery.org/style-guide/) for instructions on how to maintain your fork and submit your code. Before we can merge any pull request, we'll also need you to sign our [contributor license agreement](https://contribute.jquery.org/cla).
 
 You can [Chat on Gitter](https://gitter.im/jquery/dev), should you have any questions. If you've never contributed to open source before, we've put together [a short guide with tips, tricks, and ideas on getting started](https://contribute.jquery.org/open-source/).
+
+## Code knowledge
+
+### Protocol-relative URLs
+
+As of 2023, we run with the default WordPress settings to formatting and cleaning URLs. If revisiting this in the future, consider the following constraints:
+
+* When accessing sites in older browsers over HTTP instead of HTTPS, references to theme assets (e.g. stylesheets) must either use the current scheme, or use a protocol-relative URL, or be an absolute path URL without protocol or hostname (`theme_root_uri`).
+
+* Intra-site links to pages and categories should generally use a path or the canonical URL.
+
+* Avoid stripping the protocol from a `clean_url` filter as various uses require a full URL:
+  * Server-side requests, such as for `downloads.wordpress.org`, must specify an explicit protocol in the URL.
+  * When building `/wp-sitemap.xml`, URLs must be full and with the canonical protocol explicitly set. Sitemaps are invalid if they contain relative URLs.
+  * When outputting `<link rel=canonical>` via `wp_head/rel_canonical`, the URL must be full and canonical. Or `rel_canonical` must be remove_action'ed replaced with a custom version that calls `esc_attr()` instead of `esc_url()` to avoid the `clean_url` filter.

plugins/jquery-actions.php

@@ -14,47 +14,34 @@
 remove_action( 'wp_head',             'wp_shortlink_wp_head', 10 );
 remove_action( 'template_redirect',   'wp_shortlink_header',  11 );
 
+// Ensure relative links remain on the current protocol
+// (such as references to theme assets and intra-site links).
+// This does not influence 'home' and 'siteurl' options, and thus
+// does not affect <link rel=canonical> and sitemap output.
+if ( @$_SERVER['HTTP_X_FORWARDED_PROTO'] == 'https' ) {
+    $_SERVER['HTTPS'] = '1';
+} elseif ( @$_SERVER['HTTP_X_FORWARDED_PROTO'] == 'http' ) {
+    $_SERVER['HTTPS'] = '0';
+}
+
 /**
- * Add rel=canonical on singular pages (API pages, and blog posts)
+ * Add rel=me link to HTML head for Mastodon domain verification
+ *
+ * Usage:
+ *
+ * Put one or more comma-separated URLs in the 'jquery_xfn_rel_me' WordPress option.
+ *
+ * Example:
+ *
+ *     'jquery_xfn_rel_me' => 'https://example.org/@foo,https://social.example/@bar'
  *
- * Derived from WordPress 6.3.1 rel_canonical:
+ * See also:
  *
- * - Avoid applying esc_url and its 'clean_url' filter so that
- *   'https://' is not stripped, and thus the URL is actually canonical.
+ * - https://docs.joinmastodon.org/user/profile/#verification
+ * - https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/rel/me
+ * - https://microformats.org/wiki/rel-me
+ * - https://gmpg.org/xfn/
  */
-function jq_rel_canonical() {
-	if ( !is_singular() ) {
-		return;
-	}
-	$id = get_queried_object_id();
-	if ( $id === 0 ) {
-		return;
-	}
-
-	$url = wp_get_canonical_url( $id );
-	if ( $url) {
-		echo '<link rel="canonical" href="' . esc_attr( $url ) . '" />' . "\n";
-	}
-}
-remove_action( 'wp_head', 'rel_canonical' );
-add_action( 'wp_head', 'jq_rel_canonical' );
-
-// Add rel=me link to HTML head for Mastodon domain verification
-//
-// Usage:
-//
-// Put one or more comma-separated URLs in the 'jquery_xfn_rel_me' WordPress option.
-//
-// Example:
-//
-//     'jquery_xfn_rel_me' => 'https://example.org/@foo,https://social.example/@bar'
-//
-// See also:
-//
-// - https://docs.joinmastodon.org/user/profile/#verification
-// - https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/rel/me
-// - https://microformats.org/wiki/rel-me
-// - https://gmpg.org/xfn/
 function jquery_xfnrelme_wp_head() {
 	$option = get_option( 'jquery_xfn_rel_me' , '' );
 	$links = $option !== '' ? explode( ',', $option ) : array();

plugins/jquery-filters.php

@@ -120,18 +120,6 @@ function jquery_unfiltered_html_for_term_descriptions() {
 	return $sortedTerms;
 }, 20, 3 );
 
-// Strip protocol from urls making them protocol agnostic.
-add_filter( 'theme_root_uri', 'strip_https', 10, 1 );
-add_filter( 'clean_url', 'strip_https', 11, 1 );
-function strip_https($url) {
-	// WordPress core updates need a protocol.
-	if ( 'downloads.wordpress.org' === parse_url( $url, PHP_URL_HOST ) ) {
-		return $url;
-	}
-
-	return preg_replace( '/^https?:/', '', $url );
-}
-
 add_filter( 'xmlrpc_wp_insert_post_data', function ( $post_data, $content_struct ) {
 	if ( $post_data['post_type'] !== 'page' ) {
 		return $post_data;