Declan Chidlow of Vale.Rocks has assembled a catalogue of HTML snippets that no specification ever blessed but that websites carried anyway, usually to negotiate with browsers, plugins, or platforms. His focus is not on formally deprecated elements but on the environmental debris of the browser wars.

The largest category is Internet Explorer appeasement. X-UA-Compatible could demand IE=edge, a specific version from 5 to 11, EmulateIE7 through EmulateIE11 modes, chrome=1 to trigger Google Chrome Frame in IE 6 through 9, or requiresActiveX=true to force IE 10 out of Metro mode. IE also offered PowerPoint-style Page-Enter and Page-Exit transitions from version 4 to 8, a Reading View opt-out via IE_RM_OFF, tap-highlight suppression, ClearType enabling for Pocket IE, and a full set of msapplication-* tags for Windows 7 pinned sites and Live Tiles, including badge polling and jump-list tasks.

Browser-specific targeting went further. IE had conditional comments with a full operator set (lt, lte, gt, gte, !, &, |). Netscape answered with JavaScript entities inside comments. The Window-target=_top meta acted as a frame buster before security headers existed.

Several tags fought third-party interference. MSSmartTagsPreventParsing opted out of IE 6 Smart Tags, which auto-hyperlinked company names until site-owner outrage killed the feature. The skype_toolbar meta disabled the notoriously unstable Skype Click to Call add-on, which Mozilla blocked outright in 2011. imagetoolbar=no suppressed IE 6's hover toolbar on images. From 2012 to 2018, Chinese sites used applicable-device and no-transform Cache-Control headers to stop Baidu's mobile transcoder, which rewrote layouts and swapped ads for Baidu Union inventory.

Search and discovery left its own fossils. PICS content labels from the W3C were dead by 2003, partly because site owners simply lied. The ICBM meta tag published geographic coordinates for the GeoURL service. The noodp and noydir robots values kept Yahoo Directory and DMoz descriptions out of search results. Google ran an AJAX crawling scheme from 2009 to 2015 based on the fragment meta tag and _escaped_fragment_ URLs. The revisit-after tag, meanwhile, was only ever consumed by a regional Canadian search engine called searchBC, yet it spread across the web as pure SEO folklore.

Mobile history adds more relics. HandheldFriendly came from AvantGo, MobileOptimized from old Windows Mobile, both displaced by the viewport meta. UC Browser and QQ Browser honored screen-orientation, x5-orientation, full-screen, and x5-fullscreen. Apple contributed apple-touch-icon, startup images, and apple-mobile-web-app-capable, with the undocumented apple-touch-fullscreen accidentally promoted to an alias after early iOS 2 demos popularized it.

Social and syndication rounds out the list: Facebook's pre-Open Graph image_src, audio_src and video_src share tags, Twitter Cards now largely ignored by X, Really Simple Discovery and pingback links for blog clients, and Chrome's CRX-less web apps, prerender hints superseded by the Speculation Rules API, and AMP's amphtml link. Chidlow notes the list is incomplete and points to the WhatWG Wiki's MetaExtensions page for the truly obscure specimens.