Creating localizable web applications
for english, add hreflang="en" to links to resources that are not going to be localized or are external to your web application.
...good: css: a[hreflang="en"]::after { content: " [en]" } html/php: <a href="http://www.mozilla.com/privacy-policy.html" hreflang="en"><?= _('privacy policy'); ?></a> don't mingle app logic and localizable content when using pure html if you decide not to use gettext on some pages (e.g.

HTMLAreaElement - Web APIs
htmlareaelement.hreflang is a domstring containing that reflects the hreflang html attribute, indicating the language of the linked resource.
... the following properties have been added: rel, rellist, media, hreflang, type, host, hostname, pathname, port, protocol, search, and hash.

HTML attribute: rel - HTML: Hypertext Markup Language
<!-- a persistent style sheet --> <link rel="stylesheet" href="default.css"> <!-- alternate style sheets --> <link rel="alternate stylesheet" href="highcontrast.css" title="high contrast"> with an hreflang attribute that differs from the document language, it indicates a translation.
... <link rel="alternate" type="application/atom+xml" href="posts.xml" title="blog"> both the hreflang and type attributes specify links to versions of the document in an alternative format and language, intended for other media: <link rel=alternate href="/fr/html/print" hreflang=fr type=text/html media=print title="french html (for printing)"> <link rel=alternate href="/fr/pdf" hreflang=fr type=application/pdf title="french pdf"> author indicates the author of the current document or article.

Gecko FAQ - Gecko Redirect 1
full support except for: elements: bdo, basefont attributes: shape attribute on the a element, abbr, axis, headers, scope-row, scope-col, scope-rowgroup, scope-colgroup, charoff, datasrc, datafld, dataformat, datapagesize, summary, event, dir, align on table columns, label attribute of option, alternate text of area elements, longdesc various metadata attributes: cite, datetime, lang, hreflang bidirectional text layout, which is only used in hebrew and arabic (ibm has begun work to add bidi support in a future release) style sheets css 1 - full support, except for: the application of styles to html column elements the ability to turn off author styles the names of certain mozilla extension pseudo-classes lack the moz- prefix css 2 - partia...

HTMLAnchorElement - Web APIs
htmlanchorelement.hreflang is a domstring that reflects the hreflang html attribute, indicating the language of the linked resource.

HTMLLinkElement - Web APIs
htmllinkelement.hreflang is a domstring representing the language code for the linked resource.

SVGAElement - Web APIs
svgaelement.hreflang is a domstring that reflects the hreflang attribute, indicating the language of the linked resource.

HTML attribute reference - HTML: Hypertext Markup Language
hreflang <a>, <area>, <link> specifies the language of the linked resource.

<a>: The Anchor element - HTML: Hypertext Markup Language
links are not restricted to http-based urls — they can use any url scheme supported by browsers: sections of a page with fragment urls pieces of media files with media fragments telephone numbers with tel: urls email addresses with mailto: urls while web browsers may not support other url schemes, web sites can with registerprotocolhandler() hreflang hints at the human language of the linked url.

<area> - HTML: Hypertext Markup Language
hreflang indicates the language of the linked resource.

<link>: The External Resource Link element - HTML: Hypertext Markup Language
hreflang this attribute indicates the language of the linked resource.

Link types - HTML: Hypertext Markup Language
otherwise, the link defines an alternative page, of one of these types: for another medium, like a handheld device (if the media attribute is set) in another language (if the hreflang attribute is set), in another format, such as a pdf (if the type attribute is set) a combination of these <a>, <area>, <link> <form> archives defines a hyperlink to a document that contains an archive link to this one.

SVG Attribute reference - SVG: Scalable Vector Graphics
ternalresourcesrequired f fill fill-opacity fill-rule filter filterres filterunits flood-color flood-opacity font-family font-size font-size-adjust font-stretch font-style font-variant font-weight format from fr fx fy g g1 g2 glyph-name glyph-orientation-horizontal glyph-orientation-vertical glyphref gradienttransform gradientunits h hanging height href hreflang horiz-adv-x horiz-origin-x i id ideographic image-rendering in in2 intercept k k k1 k2 k3 k4 kernelmatrix kernelunitlength kerning keypoints keysplines keytimes l lang lengthadjust letter-spacing lighting-color limitingconeangle local m marker-end marker-mid marker-start markerheight markerunits markerwidth mask maskcontentunits maskunits mathema...

<a> - SVG: Scalable Vector Graphics
value type: <url> ; default value: none; animatable: yes hreflang the human language of the url or url fragment that the hyperlink points to.