HTML 5

Welcome back to my semi-regular column, "The Road to HTML 5," where I'll try to explain some of the new elements, attributes, and other features in the upcoming HTML 5 specification.

The feature of the day is character encoding, specifically how to determine the character encoding of an HTML document. I am never happier than when I am writing about character encoding. But first, here is my standard "elevator pitch" description of what character encoding is:

When you think of "text," you probably think of "characters and symbols I see on my computer screen." But computers don't deal in characters and symbols; they deal in bits and bytes. Every piece of text you've ever seen on a computer screen is actually stored in a particular character encoding. There are many different character encodings, some optimized for particular languages like Russian or Chinese or English, and others that can be used for multiple languages. Very roughly speaking, the character encoding provides a mapping between the stuff you see on your screen and the stuff your computer actually stores in memory and on disk.

In reality, it's more complicated than that. Many characters are common to multiple encodings, but each encoding may use a different sequence of bytes to actually store those characters in memory or on disk. So you can think of the character encoding as a kind of decryption key for the text. Whenever someone gives you a sequence of bytes and claims it's "text," you need to know what character encoding they used so you can decode the bytes into characters and display them (or process them, or whatever).

— source

And once again, I'll repeat my standard set of background links for those of you who don't know anything about character encoding. You must read Joel Spolsky's The Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets (No Excuses!) You should read Tim Bray's three-part series, On the Goodness of Unicode, On Character Strings, and Characters vs. Bytes, and anything written by Martin Dürst.

I should also point out that you should always specify a character encoding on every HTML page you serve. Not specifying an encoding can lead to security vulnerabilities.

So, how does your browser actually determine the character encoding of the stream of bytes that a web server sends? If you're familiar with HTTP headers, you may have seen a header like this:

Content-Type: text/html; charset="utf-8"

Briefly, this says that the web server thinks it's sending you an HTML document, and that it thinks the document uses the UTF-8 character encoding. Unfortunately, in the whole magnificent soup of the world wide web, very few authors actually have control over their HTTP server. Think Blogger: the content is provided by individuals, but the servers are run by Google. So HTML 4 provided a way to specify the character encoding in the HTML document itself. You've probably seen this too:

<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">

Briefly, this says that the web author thinks they have authored an HTML document using the UTF-8 character encoding. Now, you could easily imagine a situation where both the server and the document provide encoding information. Furthermore, they might not match (especially if they're run by different people). So which one wins? Well, there's a precedence order in case the document is served with conflicting information.

This is what HTML 4.01 has to say about the precedence order for determining the character encoding:

1. User override (e.g. the user picked an encoding from a menu in their browser).
2. An HTTP "charset" parameter in a "Content-Type" field.
3. A META declaration with an "http-equiv" attribute set to "Content-Type" and a value set for "charset".
4. The charset attribute set on an element that designates an external resource.
5. Unspecified heuristic analysis.

And this is what HTML 5 has to say about it. I won't quote the whole thing here, but suffice to say it's a 7-step algorithm; step 4 has 2 sub-steps, the first of which has 7 branches, one of which has 8 sub-steps, one of which actually links to a separate algorithm that itself has 7 steps... It goes on like that for a while. The gist of it is

1. User override.
2. An HTTP "charset" parameter in a "Content-Type" field.
3. A Byte Order Mark before any other data in the HTML document itself.
4. A META declaration with a "charset" attribute.
5. A META declaration with an "http-equiv" attribute set to "Content-Type" and a value set for "charset".
6. Unspecified heuristic analysis.

...and then...

1. Normalize the given character encoding string according to the Charset Alias Matching rules defined in Unicode Technical Standard #22.
2. Override some problematic encodings, i.e. intentionally treat some encodings as if they were different encodings. The most common override is treating US-ASCII and ISO-8859-1 as Windows-1252, but there are several other encoding overrides listed in this table. As the specification notes, "The requirement to treat certain encodings as other encodings according to the table above is a willful violation of the W3C Character Model specification."

Two things should leap out at you here. First, WTF is a <meta charset> attribute? Well, it's exactly what it sounds like. It looks like this:

<meta charset=UTF-8>

I was able to find only scattered discussion about this attribute on the WHATWG mailing list.

• March 2006: Internet character encoding declaration, specifically this post by Lachlan Hunt which laid out the requirements for "paving the cowpaths" of common authoring mistakes.
• June 2007: Internal character encoding declaration, Drop UTF-32, and UTF and BOM terminology

The best explanation of the new <meta charset> attribute was given a few months later, in an unrelated thread, on a separate mailing list. Andrew Sidwell explains:

The rationale for the <meta charset=""> attribute combination is that UAs already implement it, because people tend to leave things unquoted, like:

<META HTTP-EQUIV=Content-Type CONTENT=text/html; charset=ISO-8859-1>

(There are even a few <meta charset> test cases if you don't believe that browsers already do this.)

Second, who the f— does the WHATWG think they are specifying "a willful violation of the W3C Character Model specification"‽ This is a fair question. As with many such questions, the answer is that HTML 5 is only codifying what browsers do already. ISO-8859-1 and Windows-1252 are very similar encodings. One place they differ is in so-called "smart quotes" and "curly apostrophes" — the pretty little typographical flourishes that authors love and that Microsoft Word (and many other editors) output by default. Many authors specify a ISO-8559-1 or US-ASCII encoding (because they copied that part of their template from somewhere else), but then they use curly quotes from the Windows-1252 encoding. This mistake is so widespread that browsers already treat ISO-8859-1 as Windows-1252. HTML 5 is just "paving the cowpaths" here.

To sum up: character encoding is complicated, and it has not been made any easier by several decades of poorly written software used by copy-and-paste–educated authors. You should always specify a character encoding on every HTML document, or bad things will happen. You can do it the hard way (HTTP Content-Type header), the easy way (<meta http-equiv> declaration), or the new way (<meta charset> attribute), but please do it. The web thanks you.

This item was originally posted at: http://blog.whatwg.org and is licensed under the MIT license