The earliest version of HTML (subsequently labeled HTML1), was deliberately kept very simple to make the task of browser developers easier. Subsequent versions of HTML will allow more advanced features. HTML2 (approximately what most browsers support today) includes the ability to embed images in documents, layout fill-in forms, and nest lists to arbitrary depths. HTML3 (currently being defined) will allow still more advanced features such as mathematical equations, tables, and figures with captions and flow-around text.
In the first phase of a HTTP transfer the browser sends a request for a document to the server. Included in this request is the description of the document being requested, as well as a list of document types that the browser is capable of handling. The Multipurpose Internet Mail Extensions (MIME) standard is used to specify the document types that the browser can handle, typically a variety of video, audio, and image formats in addition to plain text and HTML. The browser is able to specify weights for each document type, in order to inform the server about the relative desirability of different document types.
In response to a query the server returns the document to the browser using one of the formats acceptable to the browser. If necessary the server can translate the document from the format it is stored in into a format acceptable to the browser. For example the server might have an image stored in the highly compressed JPEG image format, and if a browser capable of displaying JPEG images requests the image it would be returned in this format. However if a browser capable of displaying images only if they are in GIF format requested the same document the server would be able to translate the image and return the (larger) GIF image. This provides a way of introducing more sophisticated document formats in future but still enabling older or less advanced browser to access the same information.
In addition to the basic "GET" transaction described above the HTTP is also able to support a number of other transaction types, such as "POST" for sending the data for fill-out forms back to the server and "PUT" which might be used in the future to allow authors to save modified versions of documents back to the server.
<protocol>://<node>/<location>The first component specifies the protocol to be used to access the document, for example, HTTP, FTP, or Gopher, etc. The second component specifies the node on the network from which the document is to be obtained, and the third component specifies the location of the document on the remote machine. The third component of the URL is passed without modification by the browser to the server, and the interpretation of this component is performed by the server, so while a documents location is often specified as a Unix-like file specification, there is no requirement that this is how it is actually interpreted by the server.
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