link rel="image_src"

For a couple of years either side of 2009, the way to tell another site which image to show alongside a link to yours was <link rel="image_src">. Yahoo! Search used it to fill a 54×98 pixel thumbnail beside a result, and Facebook read it when someone shared a URL. It arrived before Open Graph existed, was proposed as a link relation, and never got any further.

Why avoid

It is not a link relation and never was. The rel registry that the HTML Standard points to for extensions still carries image_src, with its status at proposed and its specification column reading Unknown — alongside a note that it is “probably redundant with rel=icon”. A browser tokenises rel, fails to recognise the keyword, and discards it, exactly as it does with shortcut.

The sites it was written for are gone. Facebook shipped the Open Graph protocol in 2010 and og:image replaced this mechanism wholesale; the Yahoo! Search thumbnail feature it fed no longer exists. Nothing in a modern stack reads the element — not a browser, not a crawler, not a social scraper.

What is left is worse than inert, because it looks authoritative. A page carrying both image_src and og:image states its preview image twice, the two can drift apart, and only one of them is ever read. The next person to change the site’s share image has two places to look and no way to tell from the markup which one matters.

Use instead

Open Graph, which every major sharer and search surface consumes:

<meta property="og:image" content="https://example.com/cover.jpg">
<meta property="og:image:alt" content="A description of the image">

Nothing else is needed. If a specific network wants a different crop, add that network’s own tag next to it rather than reviving a generic one.

Detectability

Fully detectable. The selector uses ~= because rel is a space-separated token set and that is how a browser parses it — the same reasoning as link/shortcut-icon, even though image_src is almost always the only token present.

The fix removes the element outright, which is safe here in a way it is not for a legacy favicon spelling: deleting this takes nothing working with it, because nothing reads it. That is the difference between a keyword a browser discards inside an attribute that still does a job, and an element whose whole purpose has been superseded.

Resources

• microformats — existing rel values — the official rel registry named by the HTML Standard; lists image_src as proposed, specification Unknown.
• HTML Standard — other link types — establishes that registry as where rel extensions are recorded, and what happens to keywords outside it.
• Niall Kennedy — “Create enhanced results on Yahoo! and Facebook with Share markup” (16 March 2009) — contemporaneous primary documentation of the mechanism and its consumers.
• The Open Graph protocol — the replacement.