What is struc­tured data?

There are several standard formats that site owners follow in order to ensure that content with struc­tured data is machine readable. These include mi­cro­for­mats, RDFa, and microdata. All three formats for data struc­tur­ing are based on semantic tagging, which is entered directly into the HTML code. Depending on the format, either tra­di­tion­al HTML at­trib­ut­es or new labeling elements can be used. The data format JSON-LD has become in­creas­ing­ly popular over the past few years; this option makes it possible to annotate a web page within a script.

Mi­cro­for­mats

The labeling format mi­cro­for­mats is used for se­man­ti­cal­ly tagging HTML and XHTML documents. Well-known HTML at­trib­ut­es, like class, rel, and rev are extracted from the website code, enabling programs like web crawlers to read out semantic in­for­ma­tion. A typical use case would be to label contact in­for­ma­tion with the mi­cro­for­mat hCard, which is in­te­grat­ed in the HTML code as class=’vcard’:

An example of common labeling for contact in­for­ma­tion in HTML:

Tagging contact in­for­ma­tion with the mi­cro­for­mat hCard

While the contact in­for­ma­tion in pure HTML markup is tagged as a div element, in­te­grat­ing the mi­cro­for­mats hCard via the HTML attribute class=‘vcard’ enables distinct semantic an­no­ta­tions for specific bits of in­for­ma­tion—like names, or­ga­ni­za­tions, or telephone numbers—to be in­cor­po­rat­ed. The advantage of this type of labeling is the easy ap­pli­ca­tion of known HTML at­trib­ut­es. Doing this limits the options of semantic an­no­ta­tions with mi­cro­for­mats to a few pre­de­fined elements. Using class at­trib­ut­es can also lead to conflicts with CSS. An API for ex­tract­ing data is also not supported by mi­cro­for­mats.

RDFa

RDFa stands for ‘resource de­scrip­tion framework in at­trib­ut­es’. The W3C rec­om­mends this format for embedding RDF state­ments in HTML, XHTML, and other XML dialects. Instead of having to rely on common HTML at­trib­ut­es, RDFa in­tro­duces new at­trib­ut­es that enable complex semantic an­no­ta­tion. The following example shows contact in­for­ma­tion as struc­tured data in RDFa format:

Ausze­ich­nung von Kon­tak­t­in­for­ma­tio­nen mit RDFa

Before tagging data with the RDFa format, the cor­re­spond­ing XML namespace has to be defined. The attribute typeof specifies which data type the subject of an RDF statement is as­so­ci­at­ed with. The attribute property de­ter­mines the predicate of a statement and also specifies char­ac­ter­is­tics for an element’s content. The ad­van­tages of data struc­tur­ing with RDFa include its high flex­i­bil­i­ty and pos­si­bil­i­ty to define custom vo­cab­u­lary. Prefixes also help keep the code compact. RDFa supports a DOM API (document object model ap­pli­ca­tion pro­gram­ming interface) that extracts a website’s struc­tured data and can also be used for in­ter­ac­tive ap­pli­ca­tions. A dis­ad­van­tage is the focus on XML and XHTML, even though RDFa can also be embedded into HTML5. A detailed guide on schema.org can be found in our tutorial on the topic. For stan­dard­ized vo­cab­u­lary of RDFa an­no­ta­tions, consult the official website.

Microdata

Microdata is a sep­a­rate­ly defined HTML5 module that can add at­trib­ut­es to existing markup language; these at­trib­ut­es are used for carrying out semantic an­no­ta­tions. As is the case with mi­cro­for­mats and RDFa, this format also uses simple at­trib­ut­es in HTML tags for assigning item features. The microdata syntax is based on a vo­cab­u­lary that allows items to be described as name/value pairs. This gives the markup format a com­pro­mise between moderate com­plex­i­ty, flex­i­bil­i­ty, and ex­pand­abil­i­ty. Microdata supports a native JSON export for trans­fer­ring data and saving struc­tured data as well as Microdata DOM API. Microdata is com­pat­i­ble with schema.org vo­cab­u­lary.

JSON-LD

JSON-LD is the newest standard for se­man­ti­cal­ly labelling website data. The acronym stands for ‘JavaScript object notation for linked data’ (in other words, the JSON-based se­ri­al­iza­tion of linked data). Google considers JSON-LD to be the simplest markup format, but doesn’t yet support all data types. Unlike mi­cro­for­mats, RDFa and Microdata, JSON-LD isn’t based on at­trib­ut­es in HTML tags. Instead, a block with JSON data is in­cor­po­rat­ed in a script of HTML code at a location of your choosing.

Initiated by market leaders Google, Bing, Yahoo!, and Yandex, the col­lab­o­ra­tive community Schema.org sets out to stan­dard­ize the semantic an­no­ta­tion of website content. Browsing through the website, users will find a uniform set of schemes for struc­tured data. Schema.org supports the data formats RDFa, Microdata, and JSON-LD.

Labeling HTML documents through semantic an­no­ta­tion requires a high level of tact. Avoiding mistakes is best done by extending a page’s source code step by step and val­i­dat­ing tags slowly as you go along. For this, Google provides a free struc­tured data testing tool. Here, site owners are able to check in­di­vid­ual code excerpts or enter the URL of a web page to check the source code for errors. The search engine giant also offers a tool, Data High­lighter, which lets users tag data directly on a web page in the browser. Relevant areas are marked with the mouse and then provided with a keyword. This method of semantic an­no­ta­tion doesn’t allow any direct labeling in the source code. The tagged areas can only be read Google and can be used for ad­di­tion­al display forms. Other search engines like Bing or Yahoo! don’t offer users the option of gathering such content.