unit24intro to markup and xhtml

unit24intro to markup and xhtml - Intro to Markup and XHTML...

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1 1 Unit 2 Intro to Markup and XHTML 2 Intro to Markup and XHTML What is the World Wide Web? 1. Indexing the World Wide Web 2. Contents of the World Wide Web Making Web Pages 1. WYSIWYG 2. Markup XHTML 1. Tags 2. Attributes 3. Entities 4. Comments 5. Hyperlinks (URLs) 6. Images 7. XHTML 1.0 Reference 8. Tables 9. Lang Attribute 0. Validators 3 What is the World Wide Web? • The World Wide Web (WWW) is a theoretical „cloud‟ of documents that are linked together through hyperlinks – Hyperlinks are connections between documents on the world wide web • A system of Internet servers that uses HTTP to transfer specially formatted documents. – The documents are formatted in a language called HTML (HyperText Mark-up Language) that supports links to other documents, as well as graphics, audio, and video files 4 What is the World Wide Web? Another view: • WWW is a service made up of many (but not all) servers that operates on the internet – Its contents are ever changing but its index is relatively constant – i.e. A very large book with changing pages but relatively constant index
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2 5 What is the World Wide Web? • Search engines such as google and yahoo are indexes for the world wide web • What happens when you search for something on google? – Unfortunately, search engines can‟t search through all the web pages in the WWW with every request (that‟s impossible) – Search engines use programs called web crawlers (or spiders) which “crawl” throughout the WWW, updating data - Indexing the WWW 6 What is the World Wide Web? • Web Crawlers – A program or script that find web pages and caches them for indexing • Caching a web page involves making a small copy of it and storing it in the search engine‟s computers (i.e. photocopying the big textbook) • Advantages of Caching: – Faster access to the stored web pages – Can store files more intelligently (i.e. like a dictionary) – It starts with a list of URL‟s to visit – For each of the URL‟s, the program • Caches it • Adds all the hyperlinks at that URL to the list of URL‟s to visit - Indexing the WWW 7 • In a real world case: – there would be a bigger list of URL‟s to visit – Each visited URL would probably contain more than just 2 links • There are some URL‟s that are useless information, how do search engines decide what to look at and what not to look at? What is the World Wide Web? - Indexing the WWW 8 What is the World Wide Web? In order to be efficient, crawlers use a combination of policies: – Selection policy : – Re-visit policy : – Politeness policy : – Parallelization policy : Trade secret! – Search engines are big business and how they search the Web is not shared
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3 9 What is the World Wide Web?
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unit24intro to markup and xhtml - Intro to Markup and XHTML...

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