Metadata and the Semantic Web - Web Standards: Mastering HTML5, CSS3, and XML - page 234

The image collects the datasets published according to the Linked Data principles and represents links between
them. The size of the bubbles corresponds to the number of triples stored in each dataset. Contributors include the
Linking Open Data community project, individuals, and organizations.
Machine-Readable Metadata
Metadata is structured data describing information about features and content of web sites. The meta tags written in (X)
HTML head sections, which do not require additional technologies, can be used to describe general data about web pages
(as mentioned earlier in Chapter 3 and will be described in detail in the next section). Semantic, machine-readable labels
can be provided as attribute values of (X)HTML or XML elements by microdata, microformats, or RDFa.
There are several metadata technologies; many apply different annotations. For example, the description of a
person can be expressed in RDFa, microdata, the vCard microformat, and further vocabularies such as FOAF or DOAC.
Special metadata such as licensing can be provided with different notations. Licensing information of images
and of the web pages containing them can be different. Providing license metadata can be beneficial to every web site,
especially the ones that have different copyright than the user content, such as image-sharing portals like Flickr [22].
Image licenses can be provided in basic markup, microdata, rel="license" microformat, and RDFa.
Several metadata technologies have multiple syntaxes. The syntax of microformats, for example, depends on the
host markup languages. In the case of RDF, some syntaxes have the same capabilities but different complexity.
Semantic Annotations
The meta Tags
In the 1990s, meta elements had a large effect on web search results. Since then, their significance has been
decreasing, partly because of the unethical tricks that have been used to manipulate search engine rankings. A good
example is keyword stuffing , which was used to load a web page with popular keywords that were not necessarily
relevant to the page content, either in the meta tags or in the content. In the latter case, the keywords were often
hidden, but the web page that contained them was indexed by search engines. Such tricks made it possible for
developers to achieve higher ranking on search results but significantly increased the number of irrelevant links on
search result lists. Although they are less important nowadays, meta tags still should be used to provide information
on web page contents for search engines.
The meta tags in HTML/XHTML can define a variety of metadata, for example, content type, author, publication
date, keywords, page content description, character encoding, and so on. These tags were introduced in HTML 2.0
and are still current.
The following attributes can be used on the meta element: content , http-equiv , name , and scheme . The first
one is the only required attribute. In HTML5, the scheme attribute is not supported on the meta element, and there is
another attribute called charset . The meta element attributes can specify the following:
• Alternatives to HTTP headers that are sent by web servers prior to the web page content.
Listing 7-1 shows an example.
Listing 7-1. Document Expiry Date Provided by the meta Tag
<meta http-equiv="expires" content="Fri, 15 October 2010 14:15:00 GMT" />
• Names and associated content attributes describing aspects of (X)HTML pages.
Listing 7-2 shows an example.
Listing 7-2. Keyword Declaration with the meta Tag
<meta name="keywords" content="standardization, accessibility" />