Web servers can offer semantic data in several ways: Content Negotiation, Web services and semantic markup in the HTML-Code of websites. The first two can provide any application directly with „understandable“ data if the reference to some ontology is clear. The latter, though it might be the least efficient way, has some advantages. One of the most important: Google „honours“ this only by a rich snippet if the information is also visible to the user.
There are stages or grades of semantic Markup. Sometimes just using HTML5-Tags as <nav> or <article> are considered semantic. But they are mostly self-referencial, telling what they are and hardly referring to some external concept. Tim Bernes Lee suggested a 5 star scheme that describes progressive levels of data publication. Roughly summarized, technically we can say that there are websites with no semantic data, using just HTML, with some sturctured and semantic data, using metatags and attributes and those with complex semantic data linked to ontologies like schema.org, Good Relations etc. that clarifies meaning and/or context.
What kind of markup you use depends on your aims. If it is classical Search Engine Optimization (SEO) you should care about your<title>-tag and description <meta name=”description” ….>. If you want to mark special offers, places, persons, media and events, using semantic SEO, then microdata, e.g. that of schema.org or much in the same way RDF Lite which is just a translation of microformats into RDF.
In Travel Websites up to now extended markup is hardly be used (status March 2014). So tripadvisor marks breadcrumbs and media, weg.de marks aggregate offers, Yahoo Travel marks mixed microdata and RDFa-data, double-referencing two similar ontologies. By this it would make it at four and a half star at best. Booking.com aims at social media, using extensive facebook and twitter markup. Most of the travel websites still have no semantic markup at all.
In most cases semantic markup is used for SEO. But it can do much more. It could , for example, feed some independent search engine for journeys hotels etc. without having to connect to some interface.