问题
I have seen conflicting information on how to best implement Open Graph namespaces. Specifically, the Open Graph website uses a few different methods, and the Facebook Open Graph examples use other methods.
Open Graph website example (using HTML prefix attribute):
<html prefix="og: http://ogp.me/ns#">
Open Graph website source code (using HTML XMLNS attribute):
<html xmlns="http://www.w3.org/1999/xhtml" xmlns:og="http://ogp.me/ns#">
Facebook Open Graph documentation (using HEAD prefix attribute):
<head prefix="og: http://ogp.me/ns# fb: http://ogp.me/ns/fb#">
Facebook Open Graph documentation #2 (using HTML XMLNS attribute):
<html xmlns="http://www.w3.org/1999/xhtml" xmlns:og="http://ogp.me/ns#"
xmlns:fb="https://www.facebook.com/2008/fbml">
What is the recommended method or does it not even matter?
回答1:
They are all equivalent and will all work.
Use prefix as it is the new recommended way and is fewer characters.
I'll get all our documentation updated to prefix.
回答2:
I tried following @Paul Tarjan's answer of using prefix in the head. However I got some Internet Explorer 8 issue. So in the end I still use the xmlns way for fb namespace:
<!DOCTYPE HTML>
<html xmlns:fb="http://ogp.me/ns/fb#">
<head prefix="og: http://ogp.me/ns# object: http://ogp.me/ns/object#">
回答3:
<html xmlns="http://www.w3.org/1999/xhtml" prefix="og: http://ogp.me/ns# fb:
http://www.facebook.com/2008/fbml"> <head>
There is the best way to validate.
回答4:
It doesn't even matter. RDFa Core Initial Context recognizes og as a widely used vocabulary prefix:
RDFa users can use these prefixes without having the obligation of defining the prefixes in the HTML code. The same list of prefixes have also been defined for JSON-LD as a JSON-LD Context at the URI
http://www.w3.org/2013/json-ld-context/rdfa11; JSON-LD users can use the@contextkey with that URI as a shorthand to use the same prefixes.
Emphasis added for clarity.
Therefore, you do not need to add a namespace to your HTML documents. More information here.
回答5:
prefix/xmlns attribute is useful to define short-hand. rdf is from xml lineage so xmlns notation should be expected to work independent of doctype detail. rdfa extends html with attributes including prefix as given by http://www.w3.org/TR/rdfa-in-html/#extensions-to-the-html5-syntax However, this requires adhering to http://dev.w3.org/html5/rdfa/rdfa-module.html Also, the tool support for rel="profile" as mentioned therein is as yet awaited. Till then, the choice is between using older doctypes, or manually placing rdf: for validation purpose or wait for validator & such tools to catch-up. The right place to make declaration is any element opening tag which is ancestor of wherever the short-hand notation will be used. Specifically for the case of og prefix, its predefined by the initial context of RDFa Core http://www.w3.org/2011/rdfa-context/rdfa-1.1.html so skipping it altogether for newer doctypes is alright. And specifically for html5 the RDFa initial context http://www.w3.org/2011/rdfa-context/html-rdfa-1.1 is loaded only after the RDFa initial context mentioned earlier.
来源：https://stackoverflow.com/questions/8235687/open-graph-namespace-declaration-html-with-xmlns-or-head-prefix