Display of Unsupported Characters

Q: How should characters be displayed if the rendering system doesn’t fully support them?

There are three main options, depending on the type of character involved. Some should not display at all (zero-width invisible characters); some should display as a visible (but blank) space; and some should be displayed with one or more genericglyphs, often referred to as “missing glyphs” or a “.notdef glyph”.

Two implementations of generic glyphs are worth calling out. One is theLast Resort Font, for which the generic glyphs are specific to the script, making it easier to identify whichfont resources might need to be installed to support the characters. The other common implementation displays the characters hex code instead of a non-specific glyph.

Q: Which characters should be displayed as a visible but blank space?

This is the easy one: all the characters that have the White_Spaceproperty, also generically known as “whitespace characters”. This set includesSPACE, of course, but also such characters as the tab control character,NO-BREAK SPACE,LINE SEPARATOR, and so on. For the full list, see the White_Space values inPropList.txt.

Q: Which characters should be displayed as invisible, if not supported?

Alldefault ignorable characters should be rendered as completely invisible (and non advancing, i.e. “zero width”), if not explicitly supported inrendering. These include:

• cursivejoiners (U+200CZWNJ, U+200DZWJ)
• bidirectional format controls (e.g. U+200ELEFT-TO-RIGHT MARK)
• the soft hyphen (U+00ADSOFT HYPHEN)
• word joiners (U+2060WORD JOINER, also U+FEFF ZWNBSP)
• thezero width space (U+200BZERO WIDTH SPACE)
• invisible math operators (e.g., U+2061FUNCTION APPLICATION)
• Jamo filler characters (e.g., U+115FHANGUL CHOSEONG FILLER)
• variation selectors

For the full list, see the Default_Ignorable_Code_Point values inDerivedCoreProperties.txt. Note that there are no White_Space chracters that have the Default_Ignorable_Code_Pointproperty.

Q: What is the intended display for variation selector sequences (including unsupported ones)?

The expectedrendering behavior for the sequence of character plus avariation selector (C+VS) is specified as follows:

• If C + VS is listed inStandardizedVariants.txt or in theIdeographic Variation Database and is supported by the rendering system, then display with the specified glyph.
• If C + FE0F is listed inemoji-variant-sequences.txt, display C asemoji, if supported.
• If C + FE0E is listed inemoji-variant-sequences.txt, display C with a regular, non-emojiglyph, if supported.
• Otherwise, display with the normal glyph for C (with no visible rendering for the VS).

If C is unsupported seeQ: How should characters be displayed if the rendering system doesn’t fully support them?

A VS sequence may also be part of agrapheme cluster, such as an emoji sequence. SeeUTS #51 Unicode Emoji for more details about emoji display.

Q: Which characters should be displayed with a missing glyph, if not supported?

All characters other than whitespace anddefault ignorable characters.

Note that recommended practice is to provide differentmissing glyphs for characters to give the user some indication of the type of character which is missing aglyph. For more information see the text under “Interpretable but Unrenderable Characters” inSection 5.3, Unknown and Missing Characters, and see theLast Resort Font.

Q: How does the recommendation not to give any visible display for a subset of default ignorable code points affect font design?

Fonts are really best viewed in the context of a wholerendering system, since other parts of that system may handle various aspects of rendering. Where a font is being designed for a rendering system that does not handle invisible characters (such asvariation selectors), then the bestglyph for them — in the absence of other support — is a zero-width invisible glyph.

Q: When would a font ever contain glyphs for invisible characters?

Rendering systems may support special display modes such as “Display Hidden”, which are intended to reveal characters that would not otherwise display.Fonts intended for such purpose would containglyphs intended for visible display ofdefault ignorablecode points that would otherwise be rendered invisibly when not supported.