From strict HTML to XHTML 1.0 in three steps

Change your DOCTYPE to XHTML 1.0 Strict.

You already know all about DOCTYPEs and you're used to seeing the HTML 4.01 Strict document type. Well, there's also a document type for XHTML 1.0 Strict, and you need to change your DOCTYPE to use it instead. Here's what it looks like:

<!DOCTYPE html
Just like the HTML
DOCTYPE, this is a public document type.
    PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
It's for the XHTML 1.0 Strict
    version of XHTML.
    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
And it has a URL pointing to
the definition of XHTML 1.0 Strict.

Add the xmlns, lang and xml:lang attributes to your <html> element.

Remember that XML can be used to define many markup languages other than XHTML. To keep all those languages straight, XML needs to know which language you're talking about when you use the element <html>(after all, someone could come along and make up their own language with XML and call it the "Hippo Tipping Markup Language," which would cause mass confusion). So, to keep things straight, the xmlns attribute specifies which language the <html> element belongs to. And what about all the rest of the elements inside the <html> element? By default, they inherit the xmlns attribute of their parent.

The <html> element also needs lang and xml:lang attributes, which specify the language being used in the XML document. Here's what your <html> opening tag should look like in XHTML:
```
<html xmlnsThe xmlns attribute is
used to identify which XML language "html" belongs to.
="http://www.w3.org/1999/xhtml"XML uses a URL as a unique
identifier for a language. If someone has written a "Hippo Tipping Markup Language" they
might have used "http://www.hippotipping.com/html" as their identifier. It doesn't matter
what is at the URL  the URL alone is enough to make it unique. 
lang="en" xml:lang="en">And we just need to specify that
we're using English. Depending on the way your XHTML is interpreted by the browser, you
may need either one of these, so it's best practice to use both.
```
All empty tags should end in " />", not ">".

This is the final, and most bizarre step of the HTML to XHTML 1.0 transformation. But it's not so mysterious if you know the background.

We've told you XHTML is stricter than HTML, and one area where it is stricter is with closing tags. In HTML, you can have an empty element without a closing tag. But in XHTML, if you aren't going to have a closing tag, you have to tell the browser about it by putting a slash before the final ">". So, take the <br> element as an example. In HTML we just write <br>. But in XHTML, we write <br/>. That little slash on the end tells the browser it shouldn't expect a closing tag, because the <br/> is all there is.

Now you might have noticed we didn't include a space before the "/>". That's because XHTML doesn't require it. However, some older browsers can't recognize "/>" without a space before the slash, so, to be backwards compatible, just put a space before your slash in " />".

From strict HTML to XHTML 1.0 in three steps

by Octavian Draganescu.

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