lib/build/import_inliner.dart - web-components - Git at Google

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 library web_components.build.import_inliner;

 import 'dart:async';
 import 'dart:collection' show LinkedHashMap;
 import 'package:barback/barback.dart';
 import 'package:code_transformers/assets.dart';
 import 'package:code_transformers/messages/build_logger.dart';
 import 'package:html/dom.dart';
 import 'package:html/dom_parsing.dart' show TreeVisitor;
 import 'package:path/path.dart' as path;
 import 'package:source_span/source_span.dart';
 import 'common.dart';
 import 'import_crawler.dart';
 import 'messages.dart';

 /// Transformer which inlines all html imports found from the entry points. This
 /// deletes all dart scripts found during the inlining, so the
 /// [ScriptCompactorTransformer] should be ran first if there are any dart files
 /// in html imports.
 class ImportInlinerTransformer extends Transformer {
   final List<String> entryPoints;
   final List<String> bindingStartDelimiters;

   ImportInlinerTransformer(
       [this.entryPoints, this.bindingStartDelimiters = const []]);

   bool isPrimary(AssetId id) {
     if (entryPoints != null) return entryPoints.contains(id.path);
     // If no entry point is supplied, then any html file under web/ or test/ is
     // an entry point.
     return (id.path.startsWith('web/') || id.path.startsWith('test/')) &&
         id.path.endsWith('.html');
   }

   apply(Transform transform) {
     var logger = new BuildLogger(transform, convertErrorsToWarnings: true);
     return new ImportInliner(transform, transform.primaryInput.id, logger,
         bindingStartDelimiters: bindingStartDelimiters).run();
   }
 }

 /// Helper class which actually does all the inlining of html imports for a
 /// single entry point.
 class ImportInliner {
   // Can be an AggregateTransform or Transform
   final transform;
   // The primary input to start from.
   final AssetId primaryInput;
   // The logger to use.
   final BuildLogger logger;
   // The start delimiters for template bindings, such as '{{' or '[['.
   final List<String> bindingStartDelimiters;

   ImportInliner(this.transform, this.primaryInput, this.logger,
       {this.bindingStartDelimiters: const []});

   Future run() {
     var crawler = new ImportCrawler(transform, primaryInput, logger);
     return crawler.crawlImports().then((imports) {
       var primaryDocument = imports[primaryInput].document;

       // Normalize urls in the entry point.
       var changed = new _UrlNormalizer(
               primaryInput, primaryInput, logger, bindingStartDelimiters)
           .visit(primaryDocument);

       // Inline things if needed, always have at least one (the entry point).
       if (imports.length > 1) {
         _inlineImports(primaryDocument, imports);
       } else if (!changed &&
           primaryDocument.querySelectorAll('link[rel="import"]').where(
                   (import) => import.attributes['type'] != 'css').length ==
               0) {
         // If there were no url changes and no imports, then we are done.
         return;
       }

       primaryDocument
           .querySelectorAll('link[rel="import"]')
           .where((import) => import.attributes['type'] != 'css')
           .forEach((element) => element.remove());

       transform.addOutput(
           new Asset.fromString(primaryInput, primaryDocument.outerHtml));
     });
   }

   void _inlineImports(
       Document primaryDocument, LinkedHashMap<AssetId, ImportData> imports) {
     // Add a hidden div at the top of the body, this is where we will inline
     // all the imports.
     var importWrapper = new Element.tag('div')..attributes['hidden'] = '';
     var firstElement = primaryDocument.body.firstChild;
     if (firstElement != null) {
       primaryDocument.body.insertBefore(importWrapper, firstElement);
     } else {
       primaryDocument.body.append(importWrapper);
     }

     // Move all scripts/stylesheets/imports into the wrapper to maintain
     // ordering.
     _moveHeadToWrapper(primaryDocument, importWrapper);

     // Add all the other imports!
     imports.forEach((AssetId asset, ImportData data) {
       if (asset == primaryInput) return;
       var document = data.document;
       // Remove all dart script tags.
       document
           .querySelectorAll('script[type="$dartType"]')
           .forEach((script) => script.remove());
       // Normalize urls in attributes and inline css.
       new _UrlNormalizer(data.fromId, asset, logger, bindingStartDelimiters)
           .visit(document);
       // Replace the import with its contents by appending the nodes
       // immediately before the import one at a time, and then removing the
       // import from the document.
       var element = data.element;
       var parent = element.parent;
       document.head.nodes
           .toList(growable: false)
           .forEach((child) => parent.insertBefore(child, element));
       document.body.nodes
           .toList(growable: false)
           .forEach((child) => parent.insertBefore(child, element));
       element.remove();
     });
   }
 }

 /// To preserve the order of scripts with respect to inlined
 /// link rel=import, we move both of those into the body before we do any
 /// inlining. We do not start doing this until the first import is found
 /// however, as some scripts do need to be ran in the head to work
 /// properly (webcomponents.js for instance).
 ///
 /// Note: we do this for stylesheets as well to preserve ordering with
 /// respect to eachother, because stylesheets can be pulled in transitively
 /// from imports.
 void _moveHeadToWrapper(Document doc, Element wrapper) {
   var foundImport = false;
   for (var node in doc.head.nodes.toList(growable: false)) {
     if (node is! Element) continue;
     var tag = node.localName;
     var type = node.attributes['type'];
     var rel = node.attributes['rel'];
     if (tag == 'link' && rel == 'import') foundImport = true;
     if (!foundImport) continue;
     if (tag == 'style' ||
         tag == 'script' &&
             (type == null || type == jsType || type == dartType) ||
         tag == 'link' && (rel == 'stylesheet' || rel == 'import')) {
       // Move the node into the wrapper, where its contents will be placed.
       // This wrapper is a hidden div to prevent inlined html from causing a
       // FOUC.
       wrapper.append(node);
     }
   }
 }

 /// Internally adjusts urls in the html that we are about to inline.
 // TODO(jakemac): Everything from here down is almost an exact copy from the
 // polymer package. We should consolidate this logic by either removing it
 // completely from polymer or exposing it publicly here and using that in
 // polymer.
 class _UrlNormalizer extends TreeVisitor {
   /// [AssetId] for the main entry point.
   final AssetId primaryInput;

   /// Asset where the original content (and original url) was found.
   final AssetId sourceId;

   /// Counter used to ensure that every library name we inject is unique.
   int _count = 0;

   /// Path to the top level folder relative to the transform primaryInput.
   /// This should just be some arbitrary # of ../'s.
   final String topLevelPath;

   /// Whether or not the normalizer has changed something in the tree.
   bool changed = false;

   // The start delimiters for template bindings, such as '{{' or '[['. If these
   // are found before the first `/` in a url, then the url will not be
   // normalized.
   final List<String> bindingStartDelimiters;

   final BuildLogger logger;

   _UrlNormalizer(AssetId primaryInput, this.sourceId, this.logger,
       this.bindingStartDelimiters)
       : primaryInput = primaryInput,
         topLevelPath = '../' * (path.url.split(primaryInput.path).length - 2);

   bool visit(Node node) {
     super.visit(node);
     return changed;
   }

   visitElement(Element node) {
     // TODO(jakemac): Support custom elements that extend html elements which
     // have url-like attributes. This probably means keeping a list of which
     // html elements support each url-like attribute.
     if (!isCustomTagName(node.localName)) {
       node.attributes.forEach((name, value) {
         if (_urlAttributes.contains(name)) {
           node.attributes[name] = _newUrl(value, node.sourceSpan);
           changed = value != node.attributes[name];
         }
       });
     }
     if (node.localName == 'style') {
       node.text = visitCss(node.text);
     } else if (node.localName == 'script' &&
         node.attributes['type'] == dartType &&
         !node.attributes.containsKey('src')) {
       changed = true;
     }
     return super.visitElement(node);
   }

   static final _url = new RegExp(r'url\(([^)]*)\)', multiLine: true);
   static final _quote = new RegExp('["\']', multiLine: true);

   /// Visit the CSS text and replace any relative URLs so we can inline it.
   // Ported from:
   // https://github.com/Polymer/vulcanize/blob/c14f63696797cda18dc3d372b78aa3378acc691f/lib/vulcan.js#L149
   // TODO(jmesserly): use csslib here instead? Parsing with RegEx is sadness.
   // Maybe it's reliable enough for finding URLs in CSS? I'm not sure.
   String visitCss(String cssText) {
     var url = spanUrlFor(sourceId, primaryInput, logger);
     var src = new SourceFile(cssText, url: url);
     return cssText.replaceAllMapped(_url, (match) {
       changed = true;
       // Extract the URL, without any surrounding quotes.
       var span = src.span(match.start, match.end);
       var href = match[1].replaceAll(_quote, '');
       href = _newUrl(href, span);
       return 'url($href)';
     });
   }

   String _newUrl(String href, SourceSpan span) {
     // We only want to parse the part of the href leading up to the first
     // folder, anything after that is not informative.
     var hrefToParse;
     var firstFolder = href.indexOf('/');
     if (firstFolder == -1) {
       hrefToParse = href;
     } else if (firstFolder == 0) {
       return href;
     } else {
       // Special case packages and assets urls.
       if (href.contains('packages/')) {
         var suffix = href.substring(href.indexOf('packages/') + 9);
         return '${topLevelPath}packages/$suffix';
       } else if (href.contains('assets/')) {
         var suffix = href.substring(href.indexOf('assets/') + 7);
         return '${topLevelPath}packages/$suffix';
       }

       hrefToParse = '${href.substring(0, firstFolder + 1)}';
     }

     // If we found a binding before the first `/`, then just return the original
     // href, we can't determine anything about it.
     if (bindingStartDelimiters.any((d) => hrefToParse.contains(d))) return href;

     Uri uri;
     // Various template systems introduce invalid characters to uris which would
     // be typically replaced at runtime. Parse errors are assumed to be caused
     // by this, and we just return the original href in that case.
     try {
       uri = Uri.parse(hrefToParse);
     } catch (e) {
       return href;
     }
     if (uri.isAbsolute) return href;
     if (uri.scheme.isNotEmpty) return href;
     if (uri.host.isNotEmpty) return href;
     if (uri.path.isEmpty) return href; // Implies standalone ? or # in URI.
     if (path.isAbsolute(hrefToParse)) return href;

     var id = uriToAssetId(sourceId, hrefToParse, logger, span);
     if (id == null) return href;

     // Build the new path, placing back any suffixes that we stripped earlier.
     var prefix =
         (firstFolder == -1) ? id.path : id.path.substring(0, id.path.length);
     var suffix = (firstFolder == -1) ? '' : href.substring(firstFolder);
     var newPath = '$prefix$suffix';

     if (newPath.startsWith('lib/')) {
       return '${topLevelPath}packages/${id.package}/${newPath.substring(4)}';
     }

     if (newPath.startsWith('asset/')) {
       return '${topLevelPath}assets/${id.package}/${newPath.substring(6)}';
     }

     if (primaryInput.package != id.package) {
       // Technically we shouldn't get there
       logger.error(internalErrorDontKnowHowToImport
               .create({'target': id, 'source': primaryInput, 'extra': ''}),
           span: span);
       return href;
     }

     var builder = path.url;
     return builder.normalize(builder.relative(builder.join('/', newPath),
         from: builder.join('/', builder.dirname(primaryInput.path))));
   }
 }

 /// Returns true if this is a valid custom element name. See:
 /// <http://w3c.github.io/webcomponents/spec/custom/#dfn-custom-element-type>
 bool isCustomTagName(String name) {
   if (name == null || !name.contains('-')) return false;
   return !invalidTagNames.containsKey(name);
 }

 /// These names have meaning in SVG or MathML, so they aren't allowed as custom
 /// tags. See [isCustomTagName].
 const invalidTagNames = const {
   'annotation-xml': '',
   'color-profile': '',
   'font-face': '',
   'font-face-src': '',
   'font-face-uri': '',
   'font-face-format': '',
   'font-face-name': '',
   'missing-glyph': '',
 };

 /// HTML attributes that expect a URL value.
 /// <http://dev.w3.org/html5/spec/section-index.html#attributes-1>
 ///
 /// Every one of these attributes is a URL in every context where it is used in
 /// the DOM. The comments show every DOM element where an attribute can be used.
 ///
 /// The _* version of each attribute is also supported, see http://goo.gl/5av8cU
 const _urlAttributes = const [
   'action',
   '_action', // in form
   'background',
   '_background', // in body
   'cite',
   '_cite', // in blockquote, del, ins, q
   'data',
   '_data', // in object
   'formaction',
   '_formaction', // in button, input
   'href',
   '_href', // in a, area, link, base, command
   'icon',
   '_icon', // in command
   'manifest',
   '_manifest', // in html
   'poster',
   '_poster', // in video
   'src',
   '_src', // in audio, embed, iframe, img, input, script, source, track,video
 ];
	// Copyright (c) 2015, the Dart project authors. Please see the AUTHORS file
	// for details. All rights reserved. Use of this source code is governed by a
	// BSD-style license that can be found in the LICENSE file.
	library web_components.build.import_inliner;

	import 'dart:async';
	import 'dart:collection' show LinkedHashMap;
	import 'package:barback/barback.dart';
	import 'package:code_transformers/assets.dart';
	import 'package:code_transformers/messages/build_logger.dart';
	import 'package:html/dom.dart';
	import 'package:html/dom_parsing.dart' show TreeVisitor;
	import 'package:path/path.dart' as path;
	import 'package:source_span/source_span.dart';
	import 'common.dart';
	import 'import_crawler.dart';
	import 'messages.dart';

	/// Transformer which inlines all html imports found from the entry points. This
	/// deletes all dart scripts found during the inlining, so the
	/// [ScriptCompactorTransformer] should be ran first if there are any dart files
	/// in html imports.
	class ImportInlinerTransformer extends Transformer {
	final List<String> entryPoints;
	final List<String> bindingStartDelimiters;

	ImportInlinerTransformer(
	[this.entryPoints, this.bindingStartDelimiters = const []]);

	bool isPrimary(AssetId id) {
	if (entryPoints != null) return entryPoints.contains(id.path);
	// If no entry point is supplied, then any html file under web/ or test/ is
	// an entry point.
	return (id.path.startsWith('web/') \|\| id.path.startsWith('test/')) &&
	id.path.endsWith('.html');
	}

	apply(Transform transform) {
	var logger = new BuildLogger(transform, convertErrorsToWarnings: true);
	return new ImportInliner(transform, transform.primaryInput.id, logger,
	bindingStartDelimiters: bindingStartDelimiters).run();
	}
	}

	/// Helper class which actually does all the inlining of html imports for a
	/// single entry point.
	class ImportInliner {
	// Can be an AggregateTransform or Transform
	final transform;
	// The primary input to start from.
	final AssetId primaryInput;
	// The logger to use.
	final BuildLogger logger;
	// The start delimiters for template bindings, such as '{{' or '[['.
	final List<String> bindingStartDelimiters;

	ImportInliner(this.transform, this.primaryInput, this.logger,
	{this.bindingStartDelimiters: const []});

	Future run() {
	var crawler = new ImportCrawler(transform, primaryInput, logger);
	return crawler.crawlImports().then((imports) {
	var primaryDocument = imports[primaryInput].document;

	// Normalize urls in the entry point.
	var changed = new _UrlNormalizer(
	primaryInput, primaryInput, logger, bindingStartDelimiters)
	.visit(primaryDocument);

	// Inline things if needed, always have at least one (the entry point).
	if (imports.length > 1) {
	_inlineImports(primaryDocument, imports);
	} else if (!changed &&
	primaryDocument.querySelectorAll('link[rel="import"]').where(
	(import) => import.attributes['type'] != 'css').length ==
	0) {
	// If there were no url changes and no imports, then we are done.
	return;
	}

	primaryDocument
	.querySelectorAll('link[rel="import"]')
	.where((import) => import.attributes['type'] != 'css')
	.forEach((element) => element.remove());

	transform.addOutput(
	new Asset.fromString(primaryInput, primaryDocument.outerHtml));
	});
	}

	void _inlineImports(
	Document primaryDocument, LinkedHashMap<AssetId, ImportData> imports) {
	// Add a hidden div at the top of the body, this is where we will inline
	// all the imports.
	var importWrapper = new Element.tag('div')..attributes['hidden'] = '';
	var firstElement = primaryDocument.body.firstChild;
	if (firstElement != null) {
	primaryDocument.body.insertBefore(importWrapper, firstElement);
	} else {
	primaryDocument.body.append(importWrapper);
	}

	// Move all scripts/stylesheets/imports into the wrapper to maintain
	// ordering.
	_moveHeadToWrapper(primaryDocument, importWrapper);

	// Add all the other imports!
	imports.forEach((AssetId asset, ImportData data) {
	if (asset == primaryInput) return;
	var document = data.document;
	// Remove all dart script tags.
	document
	.querySelectorAll('script[type="$dartType"]')
	.forEach((script) => script.remove());
	// Normalize urls in attributes and inline css.
	new _UrlNormalizer(data.fromId, asset, logger, bindingStartDelimiters)
	.visit(document);
	// Replace the import with its contents by appending the nodes
	// immediately before the import one at a time, and then removing the
	// import from the document.
	var element = data.element;
	var parent = element.parent;
	document.head.nodes
	.toList(growable: false)
	.forEach((child) => parent.insertBefore(child, element));
	document.body.nodes
	.toList(growable: false)
	.forEach((child) => parent.insertBefore(child, element));
	element.remove();
	});
	}
	}

	/// To preserve the order of scripts with respect to inlined
	/// link rel=import, we move both of those into the body before we do any
	/// inlining. We do not start doing this until the first import is found
	/// however, as some scripts do need to be ran in the head to work
	/// properly (webcomponents.js for instance).
	///
	/// Note: we do this for stylesheets as well to preserve ordering with
	/// respect to eachother, because stylesheets can be pulled in transitively
	/// from imports.
	void _moveHeadToWrapper(Document doc, Element wrapper) {
	var foundImport = false;
	for (var node in doc.head.nodes.toList(growable: false)) {
	if (node is! Element) continue;
	var tag = node.localName;
	var type = node.attributes['type'];
	var rel = node.attributes['rel'];
	if (tag == 'link' && rel == 'import') foundImport = true;
	if (!foundImport) continue;
	if (tag == 'style' \|\|
	tag == 'script' &&
	(type == null \|\| type == jsType \|\| type == dartType) \|\|
	tag == 'link' && (rel == 'stylesheet' \|\| rel == 'import')) {
	// Move the node into the wrapper, where its contents will be placed.
	// This wrapper is a hidden div to prevent inlined html from causing a
	// FOUC.
	wrapper.append(node);
	}
	}
	}

	/// Internally adjusts urls in the html that we are about to inline.
	// TODO(jakemac): Everything from here down is almost an exact copy from the
	// polymer package. We should consolidate this logic by either removing it
	// completely from polymer or exposing it publicly here and using that in
	// polymer.
	class _UrlNormalizer extends TreeVisitor {
	/// [AssetId] for the main entry point.
	final AssetId primaryInput;

	/// Asset where the original content (and original url) was found.
	final AssetId sourceId;

	/// Counter used to ensure that every library name we inject is unique.
	int _count = 0;

	/// Path to the top level folder relative to the transform primaryInput.
	/// This should just be some arbitrary # of ../'s.
	final String topLevelPath;

	/// Whether or not the normalizer has changed something in the tree.
	bool changed = false;

	// The start delimiters for template bindings, such as '{{' or '[['. If these
	// are found before the first `/` in a url, then the url will not be
	// normalized.
	final List<String> bindingStartDelimiters;

	final BuildLogger logger;

	_UrlNormalizer(AssetId primaryInput, this.sourceId, this.logger,
	this.bindingStartDelimiters)
	: primaryInput = primaryInput,
	topLevelPath = '../' * (path.url.split(primaryInput.path).length - 2);

	bool visit(Node node) {
	super.visit(node);
	return changed;
	}

	visitElement(Element node) {
	// TODO(jakemac): Support custom elements that extend html elements which
	// have url-like attributes. This probably means keeping a list of which
	// html elements support each url-like attribute.
	if (!isCustomTagName(node.localName)) {
	node.attributes.forEach((name, value) {
	if (_urlAttributes.contains(name)) {
	node.attributes[name] = _newUrl(value, node.sourceSpan);
	changed = value != node.attributes[name];
	}
	});
	}
	if (node.localName == 'style') {
	node.text = visitCss(node.text);
	} else if (node.localName == 'script' &&
	node.attributes['type'] == dartType &&
	!node.attributes.containsKey('src')) {
	changed = true;
	}
	return super.visitElement(node);
	}

	static final _url = new RegExp(r'url\(([^)]*)\)', multiLine: true);
	static final _quote = new RegExp('["\']', multiLine: true);

	/// Visit the CSS text and replace any relative URLs so we can inline it.
	// Ported from:
	// https://github.com/Polymer/vulcanize/blob/c14f63696797cda18dc3d372b78aa3378acc691f/lib/vulcan.js#L149
	// TODO(jmesserly): use csslib here instead? Parsing with RegEx is sadness.
	// Maybe it's reliable enough for finding URLs in CSS? I'm not sure.
	String visitCss(String cssText) {
	var url = spanUrlFor(sourceId, primaryInput, logger);
	var src = new SourceFile(cssText, url: url);
	return cssText.replaceAllMapped(_url, (match) {
	changed = true;
	// Extract the URL, without any surrounding quotes.
	var span = src.span(match.start, match.end);
	var href = match[1].replaceAll(_quote, '');
	href = _newUrl(href, span);
	return 'url($href)';
	});
	}

	String _newUrl(String href, SourceSpan span) {
	// We only want to parse the part of the href leading up to the first
	// folder, anything after that is not informative.
	var hrefToParse;
	var firstFolder = href.indexOf('/');
	if (firstFolder == -1) {
	hrefToParse = href;
	} else if (firstFolder == 0) {
	return href;
	} else {
	// Special case packages and assets urls.
	if (href.contains('packages/')) {
	var suffix = href.substring(href.indexOf('packages/') + 9);
	return '${topLevelPath}packages/$suffix';
	} else if (href.contains('assets/')) {
	var suffix = href.substring(href.indexOf('assets/') + 7);
	return '${topLevelPath}packages/$suffix';
	}

	hrefToParse = '${href.substring(0, firstFolder + 1)}';
	}

	// If we found a binding before the first `/`, then just return the original
	// href, we can't determine anything about it.
	if (bindingStartDelimiters.any((d) => hrefToParse.contains(d))) return href;

	Uri uri;
	// Various template systems introduce invalid characters to uris which would
	// be typically replaced at runtime. Parse errors are assumed to be caused
	// by this, and we just return the original href in that case.
	try {
	uri = Uri.parse(hrefToParse);
	} catch (e) {
	return href;
	}
	if (uri.isAbsolute) return href;
	if (uri.scheme.isNotEmpty) return href;
	if (uri.host.isNotEmpty) return href;
	if (uri.path.isEmpty) return href; // Implies standalone ? or # in URI.
	if (path.isAbsolute(hrefToParse)) return href;

	var id = uriToAssetId(sourceId, hrefToParse, logger, span);
	if (id == null) return href;

	// Build the new path, placing back any suffixes that we stripped earlier.
	var prefix =
	(firstFolder == -1) ? id.path : id.path.substring(0, id.path.length);
	var suffix = (firstFolder == -1) ? '' : href.substring(firstFolder);
	var newPath = '$prefix$suffix';

	if (newPath.startsWith('lib/')) {
	return '${topLevelPath}packages/${id.package}/${newPath.substring(4)}';
	}

	if (newPath.startsWith('asset/')) {
	return '${topLevelPath}assets/${id.package}/${newPath.substring(6)}';
	}

	if (primaryInput.package != id.package) {
	// Technically we shouldn't get there
	logger.error(internalErrorDontKnowHowToImport
	.create({'target': id, 'source': primaryInput, 'extra': ''}),
	span: span);
	return href;
	}

	var builder = path.url;
	return builder.normalize(builder.relative(builder.join('/', newPath),
	from: builder.join('/', builder.dirname(primaryInput.path))));
	}
	}

	/// Returns true if this is a valid custom element name. See:
	/// <http://w3c.github.io/webcomponents/spec/custom/#dfn-custom-element-type>
	bool isCustomTagName(String name) {
	if (name == null \|\| !name.contains('-')) return false;
	return !invalidTagNames.containsKey(name);
	}

	/// These names have meaning in SVG or MathML, so they aren't allowed as custom
	/// tags. See [isCustomTagName].
	const invalidTagNames = const {
	'annotation-xml': '',
	'color-profile': '',
	'font-face': '',
	'font-face-src': '',
	'font-face-uri': '',
	'font-face-format': '',
	'font-face-name': '',
	'missing-glyph': '',
	};

	/// HTML attributes that expect a URL value.
	/// <http://dev.w3.org/html5/spec/section-index.html#attributes-1>
	///
	/// Every one of these attributes is a URL in every context where it is used in
	/// the DOM. The comments show every DOM element where an attribute can be used.
	///
	/// The _* version of each attribute is also supported, see http://goo.gl/5av8cU
	const _urlAttributes = const [
	'action',
	'_action', // in form
	'background',
	'_background', // in body
	'cite',
	'_cite', // in blockquote, del, ins, q
	'data',
	'_data', // in object
	'formaction',
	'_formaction', // in button, input
	'href',
	'_href', // in a, area, link, base, command
	'icon',
	'_icon', // in command
	'manifest',
	'_manifest', // in html
	'poster',
	'_poster', // in video
	'src',
	'_src', // in audio, embed, iframe, img, input, script, source, track,video
	];