Add "Locale uses in Fonts" document

third_party/WebKit/Source/platform/fonts/LocaleInFonts.md

+# Locale uses in Fonts
+
+This document summarizes how locales are used in fonts.
+
+[TOC]
+
+## Parsing the Language Tag
+
+The [lang attribute] spec defines to use [BCP 47] as the language tag.
+Since ICU Locale class is not fully compatbile with [BCP 47],
+Blink uses its own simple parser
+in [platform/text/LocaleToScriptMapping.cpp](../text/LocaleToScriptMapping.cpp).
+
+`LayoutLocale::get()` parses [BCP 47] language tags
+and provides methods related with fonts and layout.
+
+`ComputedStyle::getFontDescription().locale()`
+computes the [language of a node] as defined in the spec.
+Note that this includes not only
+the value of the `lang` attribute of the element and its ancestors,
+but also the language of the document from the [content-language] header.
+Refer to the [language of a node] spec for more details.
+
+The [language of a node] could still be unknown.
+`localeOrDefault()` gives you the default language in such case.
+The default language is what Chrome uses for its UI,
+which is passed to the renderer through `--lang` command line argument.
+This could be the same or different from the language of the platform.
+
+Note that `ComputedStyle::locale()` is an `AtomicString`
+for the style system to work without special casing,
+while `FontDescription::locale()` is a pointer to `LayoutLocale`.
+
+[lang attribute]: https://html.spec.whatwg.org/multipage/dom.html#the-lang-and-xml:lang-attributes
+[BCP 47]: https://tools.ietf.org/html/bcp47
+[language of a node]: https://html.spec.whatwg.org/multipage/dom.html#language
+[content-language]: https://html.spec.whatwg.org/multipage/semantics.html#pragma-set-default-language
+
+## Generic Family
+
+Users can configure their preferred fonts for [generic-family]
+using the [Advanced Font Settings].
+Blink has this settings in `GenericFontFamilySettings`.
+In this class, each [generic-family] has a `ScriptFontFamilyMap`,
+which is a map to fonts with `UScriptCode` as the key.
+
+To look up the font to use for a [generic-family],
+Blink uses the following prioritized list to determine the script.
+
+1. The [language of a node] as defined in HTML, if known.
+2. The default language.
+
+This result is available at `ComputedStyle::getFontDescription().localeOrDefault().script()`.
+
+[generic-family]: https://drafts.csswg.org/css-fonts-3/#generic-family-value
+[Advanced Font Settings]: https://chrome.google.com/webstore/detail/advanced-font-settings/caclkomlalccbpcdllchkeecicepbmbm
+
+## System Font Fallback
+
+[CSS Fonts] defines a concept of [system font fallback],
+though its behavior is UA dependent.
+
+As Blink tries to match the font fallback behavior
+to the one in the platform,
+the logic varies by platforms.
+While the complete logic varies by platforms,
+we try to share parts of the logic where possible.
+
+[CSS Fonts]: https://drafts.csswg.org/css-fonts-3/
+[system font fallback]: https://drafts.csswg.org/css-fonts-3/#system-font-fallback
+
+### Unified Han Ideographs
+
+As seen in [CJK Unified Ideographs code charts] in Unicode,
+glyphs of Han Ideographs vary by locales.
+
+To render correct glyphs,
+the system font fallback uses the following prioritized list of locales.
+
+1. The [language of a node] as defined in HTML, if known.
+2. The list of languages the browser sends in the [Accept-Language] header.
+3. The default language.
+4. The system locale (Windows only.)
+
+The prioritized list alone may not help the Unified Han Ideographs.
+For instance, when the top of the list is "en-US",
+it gives no clue to choose the correct font for the Unified Han Ideographs.
+For this purpose,
+`LayoutLocale::hasScriptForHan()` determines whether
+the locale can choose the correct font for the Unified Han Ideographs or not.
+
+When the system font fallback needs to determine the font
+for a Unified Han Ideograph,
+it uses `scriptForHan()` of the first locale in the prioritized list
+that has `hasScriptForHan()` true.
+
+`scriptForHan()` may be different from `script()`,
+in cases such as "en-JP", which indicates an English user in Japan.
+Such locale is not major but is not rare either.
+Some organizations are known to require to use English versions of the OS,
+but their region is not US.
+
+The `script()` of "en-JP" is Latin for the [generic-family] to work correctly,
+but its `scriptForHan()` can indicate that
+the user prefers Japanese variants of glyphs for the Unified Han Ideographs.
+
+There are multiple cases where such locale can appear in the list:
+
+* A site can use such language tag in HTML or in the [content-language] header
+when its UI is in English,
+but knows that the user is in Japan,
+either by IP address, user preferences of the logged on user,
+or any other methods.
+* The system (e.g., Windows) can produce such language tag
+when its language and region are set differently.
+
+This algorithm is currently used in Windows and Linux.
+Android, before N, does not have the language settings and thus
+unable to provide the [Accept-Language] list for this algorithm to consume,
+but Android N Preview supports multi-locale
+and the work to feed the list from OS to the [Accept-Language] list is going on.
+Mac relies on Core Graphics to do the job.
+
+The prioritized list is not consistent across platforms today
+and this is being addressed.
+
+[CJK Unified Ideographs code charts]: http://unicode.org/charts/PDF/U4E00.pdf
+[Accept-Language]: https://tools.ietf.org/html/rfc7231#section-5.3.5