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[Bug 7918] New: prefetching: allow site to deny

From: <bugzilla@wiggum.w3.org>
Date: Wed, 14 Oct 2009 15:45:41 +0000
To: public-html-bugzilla@w3.org
Message-ID: <bug-7918-2486@http.www.w3.org/Bugs/Public/>

           Summary: prefetching: allow site to deny
           Product: HTML WG
           Version: unspecified
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P3
         Component: HTML5 spec proposals
        AssignedTo: dave.null@w3.org
        ReportedBy: Nick_Levinson@yahoo.com
         QAContact: public-html-bugzilla@w3.org
                CC: ian@hixie.ch, mike@w3.org, public-html@w3.org

Prefetching should be deniable, because it demands bandwidth, especially when
dynamism makes accurately predicting what will be fetched unlikely. Most site
owners will happily keep speeds up but some will want to control costs.

Prefetching is good for visitors. If the specific predictions are highly
accurate, and I imagine they would be on most sites, the bandwidth penalty is
small and probably easily overcome by visitor satisfaction leading to stiktion,
repeat visits, and a positive reputation for new visitors.

But the cost objection is right regarding sites where the arrangement of pages
is less obvious, and especially where a page is dynamic, leading to more
prefetch demands that may generate error pages or hits that don't correlate
with visitors' actual interests, so that visitors don't even glance at much
that has been prefetched at the site owner's expense. For example, if a visitor
is reading a series of pages that have "Next Page ->" links and the link is to
a script that generates a page based on how long the visitor stayed at the
current page, the prefetch would probably be erroneous.

A link supporting prefetch being absent is not a bar to prefetching (see
section 4.3.1), so the absence of the link is not sufficient protection.

Two solutions are proposed, one easier and the other more specific, and not
mutually exclusive.

The easier solution would be a local file that could deny all prefetching
sitewide. The file should be modeled on robots.txt, should be named use.txt,
should be commentable like robots.txt, and should have one noncomment line
strippable of any leading whitespace:

prefetch no

The UA, before prefetching anything at a domain, should be required to check
for that file and its command.

The reason for naming the file use.txt rather than prefetch.txt is to support
growth of the file for other purposes that may be offered or prescribed in the
future. If it is desired to add utility to use.txt before any other terms are
semanticized or reserved, private use could be supported for any term beginning
with "x-" or "data-" (your choice of which to adopt).

The more specific solution would be a per-page or per-element denial of
prefetching by a link, a, or area element. An attribute could be
rel="noprefetch" (i.e., don't prefetch the URL in the href), rev="noprefetch"
(i.e., don't prefetch the page bearing the link element), or either
noprefetch="true" (false being trivial) or prefetch="false" (true being

I'll likely add the rel to RelExtensions soon but the other solutions don't go

If what is to be prefetched is from another website, one website should not be
permitted to bar prefetching from another website. The website from which the
resource is to be prefetched would have the option of denying prefetching and
the UA would have to recognize that bar. For example, if a user visits
example.com and the UA, whether parsing example.com or not, sees fit to
prefetch from example.org, only example.org could bar the prefetching from

If a UA wants to prefetch two items for sequential use and if the first is
denied the second would not be prefetched before the first is fetched, but if
the first is allowed and the second is denied the UA would prefetch the first
and fetch but not prefetch the second.

If a UA wants to prefetch two items for joint or parallel purposing and one is
denied and the other is allowed, I don't know whether there should be an
HTML5-prescribed default whereby just one or neither is prefetched or the UA
should decide whether to prefetch just one or neither.

Thank you.


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Received on Wednesday, 14 October 2009 15:45:45 UTC

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