Do I really need to encode '&' as '&am

问题:

I\'m using an \'&\' symbol with HTML5 and UTF-8 in my site\'s <title>. Google shows the ampersand fine on its SERPs, as do all the browsers in their titles.

http://validator.w3.org is giving me this:

& did not start a character reference. (& probably should have been escaped as &.)

Do I really need to do &?

I\'m not fussed about my pages validating for the sake of validating, but I\'m curious to hear people\'s opinions on this and if it\'s important and why.

回答1:

Yes. Just as the error said, in HTML, attributes are #PCDATA meaning they\'re parsed. This means you can use character entities in the attributes. Using & by itself is wrong and if not for lenient browsers and the fact that this is HTML not XHTML, would break the parsing. Just escape it as & and everything would be fine.

HTML5 allows you to leave it unescaped, but only when the data that follows does not look like a valid character reference. However, it\'s better just to escape all instances of this symbol than worry about which ones should be and which ones don\'t need to be.

Keep this point in mind; if you\'re not escaping & to &, it\'s bad enough for data that you create (where the code could very well be invalid), you might also not be escaping tag delimiters, which is a huge problem for user-submitted data, which could very well lead to HTML and script injection, cookie stealing and other exploits.

Please just escape your code. It will save you a lot of trouble in the future.

回答2:

Validation aside, the fact remains that encoding certain characters is important to an HTML document so that it can render properly and safely as a web page.

Encoding & as & under all circumstances, for me, is an easier rule to live by, reducing the likelihood of errors and failures.

Compare the following: which is easier? which is easier to bugger up?

Methodology 1

1. Write some content which includes ampersand characters.
2. Encode them all.

Methodology 2

(with a grain of salt, please ;) )

1. Write some content which includes a ampersand characters.
2. On a case-by-case basis, look at each ampersand. Determine if:
   • It is isolated, and as such unambiguously an ampersand. eg. volt & amp
      > In that case don\'t bother encoding it.
   • It is not isolated, but you feel it is nonetheless unambiguous, as the resulting entity does not exist and will never exist since the entity list could never evolve. eg amp&volt
      > In that case don\'t bother encoding it.
   • It is not isolated, and ambiguous. eg. volt&amp
      > Encode it.

??

回答3:

I’ve researched this thoroughly and wrote about my findings here: http://mathiasbynens.be/notes/ambiguous-ampersands

I’ve also created an online tool that you can use to check your markup for ambiguous ampersands or character references that don’t end with a semicolon, both of which are invalid. (No HTML validator currently does this correctly.)

回答4:

HTML5 rules are different from HTML4. It\'s not required in HTML5 - unless the ampersand looks like it starts a parameter name. \"&copy=2\" is still a problem, for example, since © is the copyright symbol.

However it seems to me that it\'s harder work to decide to encode or not to encode depending on the following text. So the easiest path is probably to encode all the time.

回答5:

I think this has turned into more of a question of \"why follow the spec when browser\'s don\'t care.\" Here is my generalized answer:

Standards are not a "present" thing. They are a "future" thing. If we, as developers, follow web standards, then browser vendors are more likely to correctly implement those standards, and we move closer to a completely interoperable web, where CSS hacks, feature detection, and browser detection are not necessary. Where we don\'t have to figure out why our layouts break in a particular browser, or how to work around that.

Specifically, if HTML5 does not require using & in your specific situation, and you\'re using an HTML5 doctype (and also expecting your users to be using HTML5-compliant browsers), then there is no reason to do it.

回答6:

Well, if it comes from user input then absolutely yes, for obvious reasons. Think if this very website didn\'t do it: the title of this question would show up as do i really need to encode ‘&’ as ‘&’?

If it\'s just something like echo \'<title>Dolce & Gabbana</title>\'; then strictly speaking you don\'t have to. It would be better, but if you don\'t no user will notice the difference.

回答7:

Could you show us what your title actually is? When I submit

<!DOCTYPE html>
<html>
<title>Dolce & Gabbana</title>
<body>
<p>am i allowed loose & mpersands?</p>
</body>
</html>

to http://validator.w3.org/ - explicitly asking it to use the experimental HTML 5 mode - it has no complaints about the &s...

回答8:

If the user passes it to you, or it will wind up in a URL, you need to escape it.

If it appears in static text on a page? All browsers will get this one right either way, you don\'t worry much about it, since it will work.

回答9:

In HTML a & marks the begin of a reference, either of a character reference or of an entity reference. From that point on the parser expects either a # denoting a character reference, or an entity name denoting an entity reference, both followed by a ;. That’s the normal behavior.

But if the reference name or just the reference opening & is followed by a white space or other delimiters like \", \', <, >, &, the ending ; and even a reference to represent a plain & can be omitted:

<p title=\"&amp;\">foo &amp; bar</p>
<p title=\"&amp\">foo &amp bar</p>
<p title=\"&\">foo & bar</p>

Only in these cases the ending ; or even the reference itself can be omitted (at least in HTML 4). I think HTML 5 requires the ending ;.

But the specification recommends to always use a reference like the character reference & or the entity reference & to avoid confusion:

Authors should use \"&\" (ASCII decimal 38) instead of \"&\" to avoid confusion with the beginning of a character reference (entity reference open delimiter). Authors should also use \"&\" in attribute values since character references are allowed within CDATA attribute values.

回答10:

Yes, you should try to serve valid code if possible.

Most browsers will silently correct this error, but there is a problem with relying on the error handling in the browsers. There is no standard for how to handle incorrect code, so it\'s up to each browser vendor to try to figure out what to do with each error, and the results may vary.

Some examples where browsers are likely to react differently is if you put elements inside a table but outside the table cells, or if you nest links inside each other.

For your specific example it\'s not likely to cause any problems, but error correction in the browser might for example cause the browser to change from standards compliant mode into quirks mode, which could make your layout break down completely.

So, you should correct errors like this in the code, if not for anything else so to keep the error list in the validator short, so that you can spot more serious problems.

回答11:

A couple of years ago, we got a report that one of our web apps wasn\'t displaying correctly in Firefox. It turned out that the page contained a tag that looked like

<div style=\"...\" ... style=\"...\">

When faced with a repeated style attribute, IE combines both of the styles, while Firefox only uses one of them, hence the different behavior. I changed the tag to

<div style=\"...; ...\" ...>

and sure enough, it fixed the problem! The moral of the story is that browsers have more consistent handling of valid HTML than of invalid HTML. So, fix your damn markup already! (Or use HTML Tidy to fix it.)

回答12:

I was checking why Image URL\'s need escaping, hence tried it in https://validator.w3.org. The explanation is pretty nice. It highlights that even URL\'s need to be escaped. [PS:I guess it will unescaped when its consumed since URL\'s need &. Can anyone clarify?]

<img alt=\"\" src=\"foo?bar=qut&qux=fop\" />

An entity reference was found in the document, but there is no reference by that name defined. Often this is caused by misspelling the reference name, unencoded ampersands, or by leaving off the trailing semicolon (;). The most common cause of this error is unencoded ampersands in URLs as described by the WDG in \"Ampersands in URLs\". Entity references start with an ampersand (&) and end with a semicolon (;). If you want to use a literal ampersand in your document you must encode it as \"&\" (even inside URLs!). Be careful to end entity references with a semicolon or your entity reference may get interpreted in connection with the following text. Also keep in mind that named entity references are case-sensitive; &Aelig; and æ are different characters. If this error appears in some markup generated by PHP\'s session handling code, this article has explanations and solutions to your problem.

回答13:

if & is used in html then you should escape it

If & is used in javascript strings e.g. an alert(\'This & that\'); or document.href you don\'t need to use it.

If you\'re using document.write then you should use it e.g. document.write(<p>this &amp; that</p>)

回答14:

It depends on the likelihood of a semicolon ending up near your &, causing it to display something quite different.

For example, when dealing with input from users (say, if you include the user-provided subject of a forum post in your title tags), you never know where they might be putting random semicolons, and it might randomly display strange entities. So always escape in that situation.

For your own static html, sure, you could skip it, but it\'s so trivial to include proper escaping, that there\'s no good reason to avoid it.

回答15:

If you\'re really talking about the static text

<title>Foo & Bar</title>

stored in some file on the hard disk and served directly by a server, then yes: it probably doesn\'t need to be escaped.

However, since there is very little HTML content nowadays that\'s completely static, I\'ll add the following disclaimer that assumes that the HTML content is generated from some other source (database content, user input, web service call result, legacy API result, ...):

If you don\'t escape a simple &, then chances are you also don\'t escape a & or a   or <b> or <script src=\"http://attacker.com/evil.js\"> or any other invalid text. That would mean that you are at best displaying your content wrongly and more likely are suspectible to XSS attacks.

In other words: when you\'re already checking and escaping the other more problematic cases, then there\'s almost no reason to leave the not-totally-broken-but-still-somewhat-fishy standalone-& unescaped.

回答16:

not sure if this is useful to anyone... I was fighting this for a while... here is a glorious regex you can use to fix all your links, javascript, content. I had to deal with a ton of legacy content that nobody wanted to correct.

Add this to your Render override in your master page or control:

Please don\'t flame me for putting this in the wrong place:

// remove the & from href=\"blaw?a=b&b=c\" and replace with & 
//in urls - this corrects any unencoded & not just those in URL\'s
// this match will also ignore any matches it finds within <script> blocks AND
// it will also ignore the matches where the link includes a javascript command like
// <a href=\"javascript:alert{\'& & &\'}\">blaw</a>
html = Regex.Replace(html, \"&(?!(?<=(?<outerquote>[\\\"\'])javascript:(?>(?!\\\\k<outerquote>|[>]).)*)\\\\k<outerquote>?)(?!(?:[a-zA-Z][a-zA-Z0-9]*|#\\\\d+);)(?!(?>(?:(?!<script|\\\\/script>).)*)\\\\/script>)\", \"&amp;\", RegexOptions.Singleline | RegexOptions.IgnoreCase);

回答17:

The link has a fairly good example of when and why you may need to escape & to &

https://jsfiddle.net/vh2h7usk/1/

Interestingly, I had to escape the character in order to represent it properly in my answer here. If I were to use the built-in code sample option (from the answer panel), I can just type in & and it appears as it should. But if I were to manually use the <code></code> element, then I have to escape in order to represent it correctly :)