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269 lines
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269 lines
10 KiB
<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta name="generator" content=
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"HTML Tidy for Mac OS X (vers 31 October 2006 - Apple Inc. build 13), see www.w3.org" />
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<title>
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The Implications of Links -- Axioms of Web architecture
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</title>
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<link href="di.css" rel="stylesheet" type="text/css" />
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<meta http-equiv="Content-Type" content="text/html" />
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</head>
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<body bgcolor="#DDFFDD" text="#000000">
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<address>
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Tim Berners-Lee
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<p>
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Date: April 1997
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</p>
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<p>
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Status: personal view only. Editing status: first draft.
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</p>
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</address>
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<p>
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<a href="./">Up to Design Issues</a>
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</p>
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<h3>
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Commentary on Web Architecture
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</h3>
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<hr />
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<h1>
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Links and Law
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</h1>
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<h3>
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<i>Preface</i>
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</h3>
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<p>
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This personal note I have put into the set of web
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architectural notes as it expresses fundamental
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understandings upon which the practical use and power of the
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web rest.
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</p>
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<p>
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The questions addressed are about the relationship of the
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hypertext forms of <i>linked</i> and <i>embedded</i> material
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to the social concepts involved such as attribution,
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endorsement, and ownership of information.
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</p>
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<p>
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Links in hypertext are new in that they can be followed
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automatically, but the concepts of reference and inclusion of
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material predate paper. There should not therefore be much
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confusion about what links imply, but as there have been some
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strange suggestions recently which would seriously damage the
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web, I write this note.
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</p>
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<h3>
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<a name="Abstract" id="Abstract">Abstract</a>
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</h3>
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<p>
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Normal hypertext links do not of themselves imply that the
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document linked to is part of, is endorsed by, or endorses,
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or has related ownership or distribution terms as the
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document linked from. However, embedding material by
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reference (sometimes called an embedding form of hypertext
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link) causes the embedded material to become a part of the
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embedding document.
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</p>
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<h2>
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<a name="sorts" id="sorts">Two sorts of link</a>
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</h2>
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<p>
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Basic HTML has three ways of linking to other material on the
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web: the hypertext link from an anchor (HTML "A" element),
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the general link with no specific source anchor within the
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document (HTML "LINK" element) and embedded objects and
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images (IMG and OBJECT). Let's call A and LINK
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"<b>normal</b>" links as they are visible to the user as a
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traversal between two documents. We'll call the thing between
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a document and an embedded image or object or subdocument
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"<b>embedding</b>" links.
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</p>
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<p>
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This distinction is an old one in hypertext. Some systems
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such Peter Brown's original "Guide" worked only by expanding
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links inline, and some (such as HTML before the IMG tag was
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introduced) worked only with normal links.
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</p>
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<h2>
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<a name="Normal" id="Normal">Normal Links</a>
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</h2>
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<table border="1" cellpadding="2">
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<tbody>
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<tr>
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<td>
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<b>The intention in the design of the web was that
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normal links should simply be references, with no
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implied meaning.</b>
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</td>
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</tr>
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</tbody>
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</table>
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<p>
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A normal hypertext link does NOT necessarily imply that
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</p>
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<ul>
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<li>One document endorses the other; or that
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</li>
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<li>One document is created by the same person as the other,
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or that
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</li>
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<li>One document is to be considered part of another.
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</li>
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</ul>
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<p>
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Typically when the user of a graphical window-oriented Web
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browser follows a normal link, a new window is created and
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the linked document is displayed in it, or the old document
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is deleted from its window and the linked document displayed
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in its place. The window system has a user interface metaphor
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that things in different windows are different objects.
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</p>
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<h3>
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Meaning in content
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</h3>
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<p>
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So the existence of the link itself does not carry meaning.
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Of course the contents of the linking document can carry
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meaning, and often does. So, if one writes "See Fred's web
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pages (link) which are way cool" that is clearly some kind of
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endorsement. If one writes "We go into this in more detail on
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our sales brochure (link)" there is an implication of common
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authorship. If one writes "Fred's message (link) was written
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out of malice and is a downright lie" one is denigrating
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(possibly libellously) the linked document. So the content of
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hypertext documents carry meaning often about the linked
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document, and one should be responsible about this. In fact,
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clarifying the relative status of the linked document is
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often helpful to the reader.
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</p>
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<h2>
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<a name="Embedded" id="Embedded">Embedded Material</a>
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</h2>
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<p>
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The relationship between a document and an image embedded in
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that document is quite different from normal link. (In some
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designs it is still refered to as a sort of link).
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</p>
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<table border="1" cellpadding="2">
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<tbody>
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<tr>
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<td>
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<b>Images, embedded objects, and background sounds and
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images are by default to be considered part of the
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document.</b>
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</td>
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</tr>
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</tbody>
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</table>
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<p>
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If I say, "To understand this you only have to read this
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article", or "This is the agreement between us", I am talking
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about a particular document. It is important that we have a
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clear picture of what is part of that document and what
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isn't. Embedded images clearly are part of the embedding
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document. The author of a document has responsibility for the
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content, even if the images he or she includes are from
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another web site.
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</p>
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<p>
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(There are issues of expectations to be set about
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availability and security from corruption of remote material,
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but I do not address these here. Here I just emphasize is
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that embedded images should be considered part of a document,
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but documents connected by a normal link should be regarded
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as separate documents.)
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</p>
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<p>
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We compose documents out of parts, and the finished work
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comprises contributions from the parts and also from the
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arrangement. It is very important that we can include remote
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parts by reference without having to make a separate local
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copy. When an embedded image (or sound) is included by
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reference to its original address (URI) this allows an
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inquirer to know that address, and hence know the current
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version of the image. It allows the owner of the image to to
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a certain extent to know and possibly to control who has
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access to that image. Also I expect in that in the future it
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will allow one to find out the owner and licence terms for
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distribution of that image, which is important for
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intellectual property rights to be respected on the Web.
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</p>
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<h4>
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Explict distinction
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</h4>
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<p>
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Advertising provides an exception to this rule: a case in
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which the embedded image is <b>not</b> part of the document.
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At risk of making ittoo easy for users to turn
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off advertizing, it would be ideal if the distinction
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were make in the markup between embeeded information which is
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or is not part of the document. This would allow, for
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example, a border to be places around an advertizement to
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allow the user to realize that it does not come from the same
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source as the text. I personally feel that this would
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be an important step forward in the integrity of the
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web. A flag like
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</p>
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<pre>
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<IMG src="banner-ad.gif" foreign>
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</pre>
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<p>
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would be fine.
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</p>
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<h2>
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<a name="User" id="User">User Interface</a>
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</h2>
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<p>
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When Web documents are presented to people, most current
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browsers (1997) make a clear distinction between embedded
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images, which are presented in the same window as the
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embedding document at the same time, and linked documents
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which never are. The window system's concept of a "Window" is
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used to convey when things are part of the same document. It
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is important for many reasons, some of which were mentioned
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above, that user interfaces continue to make this
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distinction.
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</p>
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<h4>
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Frames
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</h4>
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<p>
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The "frames" of HTML unfortunately provide an interface which
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is less clear. The parts of the document do appear with the
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same window, but because within a single frame (subsection of
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a window) one can follow hypertext links replacing content
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with a separate document, it is easy to create the impression
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that the owner of the surrounding frames is in fact
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responsible for the defining document. It is possible that
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work by the HTML community can produce explict markup (such
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as the "foreign" flag above) for conveying, when frames are
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used, which parts of the screen are considered to be the same
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document. In the mean time, it is appropriate for content
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providers so make efforts to ensure by the design of (and/or
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statements on) their web pages that users are not left with
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the illusion that information within an embedded frame is
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part of their document when it is really not.
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</p>
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<p>
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<i>Next: Some dangerous <a href="LinkMyths.html"><b>Myths</b>
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about Links</a></i>
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</p>
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<hr />
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<p>
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<i>A reminder that this is personal opinion, not related to
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W3C or MIT policy. I reserve the right to rephrase this if
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misunderstandings occur, as its always difficult to express
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this sort of thing to a mixed and varied audience.</i>
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</p>
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<hr />
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<p>
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<a href="Metadata.html">Next: Metadata architecture</a>
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</p>
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<p>
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<a href="Overview.html">Up to Design Issues</a>
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</p>
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<p>
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<a href="../People/Berners-Lee">Tim BL</a>
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</p>
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</body>
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</html>
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