harloo…
I just started know about web pages design…
so have many things that confussing in my mind.
1 ) What is the difference between HTML and XML?
2 ) What is the difference between logical elements and physical elements?Can provide some examples?
thanks so much….
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The most salient difference between HTML and XML is that HTML describes presentation and XML describes content. An HTML document rendered in a web browser is human readable. XML is aimed toward being both human and machine readable.
XML syntax differs from HTML
1.New tags may be defined at will
2.Tags may be nested to arbitrary depth
3.May contain an optional description of its grammar
HTML – short for Hypertext Markup Language, is the predominant markup language for the creation of web pages. It provides a means to describe the structure of text-based information in a document — by denoting certain text as headings, paragraphs, lists, and so on — and to supplement that text with interactive forms, embedded images, and other objects. HTML is written in the form of labels (known as tags), surrounded by less-than (<) and greater-than signs (>). HTML can also describe, to some degree, the appearance and semantics of a document, and can include embedded scripting language code which can affect the behavior of web browsers and other HTML processors.
XML – The Extensible Markup Language is a general-purpose markup language.[1] It is classified as an extensible language because it allows its users to define their own tags. Its primary purpose is to facilitate the sharing of data across different information systems, particularly via the Internet. It is a simplified subset of the Standard Generalized Markup Language (SGML), and is designed to be relatively human-legible. By adding semantic constraints, application languages can be implemented in XML. These include XHTML,[3] RSS, MathML, GraphML, Scalable Vector Graphics, MusicXML, and thousands of others. Moreover, XML is sometimes used as the specification language for such application languages.
They are two different web-based languages, for applications and internet.
HTML is for example this page, it is made up of HTML.
XML is for RSS.
HTML, short for Hypertext Markup Language, is the predominant markup language for the creation of web pages. It provides a means to describe the structure of text-based information in a document — by denoting certain text as headings, paragraphs, lists, and so on — and to supplement that text with interactive forms, embedded images, and other objects. HTML is written in the form of labels (known as tags), surrounded by less-than (<) and greater-than signs (>). HTML can also describe, to some degree, the appearance and semantics of a document, and can include embedded scripting language code which can affect the behavior of web browsers and other HTML processors.
The Extensible Markup Language (XML) is a general-purpose markup language.[1] It is classified as an extensible language because it allows its users to define their own tags. Its primary purpose is to facilitate the sharing of data across different information systems, particularly via the Internet.[2]
It is a simplified subset of the Standard Generalized Markup Language (SGML), and is designed to be relatively human-legible. By adding semantic constraints, application languages can be implemented in XML. These include XHTML,[3] RSS, MathML, GraphML, Scalable Vector Graphics, MusicXML, and thousands of others. Moreover, XML is sometimes used as the specification language for such application languages.
For the most part, elements and attributes in HTML can be divided into two groups: physical and logical. A logical HTML element or attribute is similar to an XML element. Logical elements and attributes describe the format of the content enclosed within the tags. For example, here the text Hello, world should be displayed with a font size of 3:
<font size="3">Hello, world</font>
The actual size of the font will depend on the browser settings and the user’s preferences. With logical elements, the Web browser will use the markup elements and attributes to identify what the content is and then display the content accordingly.
Physical elements and attributes do not give the user any options as to how content is displayed—they define exactly what the content will look like. Rewriting our font size example using a physical attribute, we have:
<font size="12 pt">Hello, world</font>
The Hello, world text will now always be displayed in 12 point type, regardless of the user’s preferences. The attribute no longer defines the content as being of a certain format that the application will interpret; it simply sets the attribute to a value that the application will use.
When you develop XML applications, you will want to define elements and attributes that give the user more control, such as logical HTML elements and attributes. These elements and attributes will be used by the application to identify the content contained within the element. Once the application understands what the content is, it can determine how to use the content based on user preferences (for example, setting the default size 3 text to 14 point text in the browser), the structure of the database (for example, in one corporation a customer’s last and first names might be saved as a single entity called CustomerName, and in another corporation the same information might be saved as LName and FName), and so on. As we create Web applications using XML throughout this book, we will use logical elements and attributes whenever possible.
The main problem we will have with building Web applications using XML is that most people are working with browsers that only understand HTML. We’ll need some way to get the non-XML browsers to view XML code as HTML code so that the pages will render properly in the browser. When cascading style sheets (CSS) were introduced, they also faced the same problem: how to render documents properly in non-CSS browsers. The ingenious solution that was used for CSS documents can also be used for XML. Let’s take a look at how CSS can work in both browsers that understand style sheets and browsers that do not.
> What is the difference between HTML and XML?
HTML and XML are both SGML derivatives. They both function similarily: they are intended to markup documents or similar data. The markup is similar: the use of a DTD, tags, attributes, and so on.
HTML is designed for web usage. The DTD specifies elements that are useful in web documents: headings, paragraphs, hyperlinks, and so on. XML is generic. You use it for whatever you want. So many markup formats are XML formats. RSS (news feeds) uses XML to markup the data. One of my development IDEs uses a configuration file to store program settings. This file uses XML. XML is a generic method of marking up data, and you can adapt it to whatever situation you want.
Which is why it is possible to have HTML that is also XML. Known as XHTML. XHTML …is… XML, and should be treated as one. But it is also HTML, because it is adapted for web usage.
> What is the difference between logical elements and physical elements?
Let’s say you are reading an essay. At the top is a title in big bold letters. Why is it in big bold letters? Because that’s the title. It could be in small underlined letters. Red letters. Blue letters. Who cares. That’s the title.
Remember how I said HTML is about document markup? Documents have headings, subheadings, paragraphs, quotes, citations, and so on. Logical elements indicate these structural components of a document.
Physical elements have no structural meaning. They are things like bold tags, or italic tags, or underline tags. What is the meaning of bold? Nothing. It just makes text bold.
Here’s the thing. HTML is only for indicating structural meaning. You can determine the look of a webpage with what is known as CSS. CSS makes stylesheets, and in the case of webpages, they can be applied to HTML. So if you have a heading in HTML, you can use CSS to make that heading look big, or red, or underlined, or whatever.
Google for “semantic HTML”. The mistake people make is they use <center> tags to center a heading. Or <b> (bold) tags to emphasize something. Or font tags to make something big or small. Don’t. Why do you center something? Because it is a heading? Then use <h1> or <h2> tags, which indicate logically that is a heading. Use CSS to choose how it looks.
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