[selectors-4] Revert accidental commit to selectors-4 from css-display-3 commit 1432231

Author: fantasai

selectors-4/Overview.bs

@@ -1982,187 +1982,35 @@ The Hyperlink Pseudo-class: '':any-link''</h3>
 <h3 id="link">
 The Link History Pseudo-classes: '':link'' and '':visited''</h3>
 
-	User agents commonly display unvisited <a href="#the-any-link-pseudo">hyperlinks</a>
-	differently from previously visited ones.
-	Selectors provides the pseudo-classes
-	<dfn id='link-pseudo'>:link</dfn> and <dfn id='visited-pseudo'>:visited</dfn>
-	to distinguish them.
-	Roughly:
-
-	* The '':link'' pseudo-class applies to links that have
+	User agents commonly display unvisited <a href="#the-any-link-pseudo">hyperlinks</a> differently from
+	previously visited ones. Selectors
+	provides the pseudo-classes <dfn id='link-pseudo'>:link</dfn> and
+	<dfn id='visited-pseudo'>:visited</dfn> to distinguish them:
+
+	<ul>
+		<li>The '':link'' pseudo-class applies to links that have
 		not yet been visited.
-	* The '':visited'' pseudo-class applies once the link has
+		<li>The '':visited'' pseudo-class applies once the link has
 		been visited by the user.
+	</ul>
 
 	After some amount of time, user agents may choose to return a
 	visited link to the (unvisited) '':link'' state.
 
-	The two states are mutually exclusive
-	(no element can ever match '':link:visited''),
-	but their actual interaction is more complex than that,
-	for privacy reasons explained below.
-
-	In full, the behavior is actually that the '':link'' pseudo-class applies to all links,
-	but the '':visited'' pseudo-class applies the following behavior and restrictions:
-
-	<div algorithm=":visited styling">
-		To <dfn>apply :visited styling</dfn> to an element |el|:
-
-		1. Check to see if |el| has a <a>relevant link</a>.
-			The <dfn for=":visited">relevant link</dfn> for an element
-			is the element itself,
-			if it's an element that would match '':link'',
-			or else the closest ancestor that would match '':link''.
-
-			If |el| does not have a <a>relevant link</a>,
-			then '':visited'' has no effect on the element.
-
-		2. Otherwise, compute |el|’s style,
-			treating only its <a>relevant link</a> as matching '':visited''
-			(and thus, not '':link'').
-			All other links must be treated as matching '':link''.
-
-		3. From the styling results,
-			record the values of the <dfn lt="allowed :visited property">allowed :visited properties</dfn>:
-
-			* 'color'
-			* 'background-color'
-			* the 'border-color' sub-properties
-			* 'outline-color'
-			* 'column-rule-color'
-			* 'fill-color'
-			* 'stroke-color'
-
-			These are |el|’s <dfn lt=":visited style">:visited styles</dfn>.
-
-		4. If |el|’s <a>relevant link</a> is actually visited,
-			then for the <a>allowed :visited properties</a>,
-			use |el|’s <a>:visited styles</a> for any purposes
-			that won't allow the document itself to tell what the style is;
-			in all other contexts,
-			use the normal, all-'':link'', styling.
-	</div>
+	The two states are mutually exclusive.
 
 	<div class="example">
-		For example, with the following common example style sheet:
-
-		<pre class=lang-css>
-		:link { color: blue; }
-		:visited { color: purple; }
-		</pre>
-
-		Visited links show up as purple only for rendering to the screen.
-		If the document uses JS to query the element’s style,
-		it will instead report ''color: blue'',
-		as if the element only matched '':link''.
+		The following selector represents links carrying class
+		<code>footnote</code> and already visited:
 
-		Similarly, if HTML's <{canvas}> element develops an ability to render HTML to the canvas with CSS styling,
-		then either the links would be styled blue
-		(because by default, <{canvas}> gives the document the ability to inspect the results),
-		or the visited links will be purple,
-		but <{canvas}> will restrict the page's ability to inspect the pixels of the result.
+		<pre>.footnote:visited </pre>
 	</div>
 
-	<div class=example>
-		The special '':visited'' behavior also means
-		that a single element might be styled by a mix of '':link'' and '':visited'' rules.
-		For example:
-
-		<pre class=lang-css>
-		:link {
-			color: blue;
-			background-image: url("unvisited.png");
-		}
-		:visited {
-			color: purple;
-			background-image: url("visited.png");
-		}
-		</pre>
-
-		With this style sheet,
-		a visited link will be colored purple,
-		but have the "unvisited.png" background,
-		because 'background-image' is not one of the <a>allowed :visited properties</a>,
-		and so whatever value it gets from a '':visited'' style
-		is not recorded in the element's <a>:visited styles</a>.
-	</div>
-
-	<div class=example>
-		Another consequence of the special '':visited'' behavior
-		is that some selectors that look reasonable
-		will never match.
-		For example:
-
-		<pre class=lang-css>
-		:visited + span { color: red; }
-		</pre>
-
-		Even tho this style sheet is applying an <a>allowed :visited property</a> to the <code>span</code> element,
-		the <code>span</code>’s <a>relevant link</a> can never be its previous sibling
-		(it can only be the <code>span</code> or one of its ancestors).
-		Since the [=apply :visited styling|:visible styling algorithm=]
-		only checks if the element's <a>relevant link</a> is '':visited''
-		(and treats all other links on the page as unvisited),
-		this selector will never actually match anything.
-	</div>
-
-	<details class=note>
-		<summary>Why does '':visited'' have this strange behavior and restrictions?</summary>
-
-		Originally, '':link'' and '':visited'' did indeed work in the simple way
-		described at the beginning of this chapter,
-		like two ordinary mutually-exclusive pseudo-classes.
-		It was eventually discovered, however,
-		that this allowed pages to determine what other sites a user had visited,
-		by listing a bunch of links off-screen
-		and using JS to tell whether they were styled with '':link'' or '':visited''.
-
-		This was bad both for user's privacy
-		(for obvious reasons, sharing a user's browsing history with everyone is bad)
-		and for their security
-		(phishing attacks could, for example, tell which bank website a user visited,
-		and render their phishing page to match that bank specifically,
-		making it more likely to fool the user).
-
-		Rendering visited links in a different style was too useful to throw away entirely,
-		so instead user agents developed the above algorithm
-		to "lie" about the style in some contexts,
-		but still rendering the link with '':visited'' styles normally,
-		without paying too much extra computation time.
-
-		The very limited list of <a>allowed :visited properties</a>
-		further limits the possibility of a page figuring things out.
-		If '':visited'' could apply any property,
-		pages could still,
-		for example,
-		apply a particular 'background-image' only when a link is visited,
-		and then record whether that image was loaded from their server,
-		giving them the exact information we were trying to hide!
-		Similarly, any layout-affecting property,
-		like 'width',
-		might affect the positions of <em>other</em> elements on the page;
-		lying about these knock-on effects to hide the styling
-		would be much more expensive for the user agent.
-		Limiting it just a handful of properties that can only apply colors
-		ensures that as little information is leakable as possible.
-	</details>
-
-	User agents may treat all links as unvisited at all times.
-	(In particular, they can offer a user preference for this,
-	allowing the user to decide whether the benefit of seeing when a link has been visited
-	is more or less important
-	than the possibility of that information being leaked to random pages on the internet.)
-
-	If the user agent uses some form of "link pre-loading"
-	to find resource urls in a stylesheet
-	and begin loading them before it's known that they'll actually be used on the page,
-	they might find a url that would only be applied by an element matching '':visited''
-	(which is impossible, as '':visited'' rules can't cause any properties to apply that load images).
-	User agents with this behavior must ensure that either
-	such urls are <em>never</em> loaded (preferable),
-	or at least that such urls are loaded at the same time
-	regardless of whether a link is visited or not.
-
+	Since it is possible for style sheet authors to abuse the :link and :visited pseudo-classes
+	to determine which sites a user has visited without the user's consent,
+	UAs may treat all links as unvisited links
+	or implement other measures to preserve the user's privacy
+	while rendering visited and unvisited links differently.
 
 <h3 id="the-local-link-pseudo">
 The Local Link Pseudo-class: '':local-link''</h3>