Accessibility/TextAttributes

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Introduction

This article is targeted to reflect a support of text attributes by Gecko accessibility API. There is no implementation in Gecko yet. This is a draft of specification how we will do it.

Proposed API

Summary

This is the internal nsIAccessibleText method (not the ATK or IA2 method -- the implementation for those will consume this method).

The following method returns a collection of text attributes at the given offset, and calculates the range where returned attributes are stretched.

 nsIPersistentProperties getTextAttributes(in long offset,
                                           out long rangeStartOffset,
                                           out long rangeEndOffset);

Parameters

offset

[in] the given offset

rangeStartOffset

[out] the start offset of the result range

rangeEndOffset

[out] the end offset of the result range

Return Value

A collection of text attributes (the list of pairs consisted from name and value).

Events

Summary

When a certain text attribute is changed then 'EVENT_TEXT_ATTRIBUTE_CHANGED' event should be fired.

Questions

Range questions

1. What is the range hierarchy: are range nested or consecutive? Let we have a text 'abcdefghi'. If ranges are nested then we have two ranges: (0,9) and (3,6). If ranges are consecutive then we have three ranges (0, 3), (3, 6) and (6, 9).
2. Should text attributes in a range be either relative to a parent (in the case of nested ranges) or previous (in the case of consecutive ranges) range or cumulative?

ATK default attribute questions

1. Should we support default attributes? IA2 hasn't their support. We could do it through an object attribute, if we really need it.
2. One thing we can do even for IA2 is specify a default value for attributes that aren't exposed. For example, we could just not expose font-style when it is "normal", and text-align when it is "left". This would save us from exposing a lot of extra attributes -- is it reasonable?

Unit questions

1. The IA2 site does not say what unit color values are exposed as -- it just says use CSS values. However, in CSS colors can be exposed in many different ways. You can use the color name, you can use a 3 digit hex value, a 6 digit hex value, 3 comma-separated unsigned shorts, .... Is it better just to get 3 comma separated unsigned shorts (R, G, B) so you can determine a name for the color yourself?
2. We have a similar problem for anything exposing a length, e.g. margin-left. How do you want the units?

Event questions

1. How will AT know what attribute has been changed? Should the getTextAttributes() method return only last changed attribute?
2. It's not clear how to track CSS style changes mapped to text attributes. Possibly some of those changes will lead accessible is recreated.
3. Are more events other than changes to whether an item is misspelled even useful?
4. There could be a lot of attribute change events -- for example, if someone selects all and then boldfaces the selection -- every object in the tree is getting an attribute change. We could reduce the number of events by removing events on descendants of something already firing the event. It could be implied by the change event on the ancestor. Is that helfpul?

List of supported attributes

Gecko specific

Attribute name	Attribute description	Default value if attribute not exposed
direction	rtl or ltr for bidi text	ltr
border	not sure how best to expose these properties, or whether to (same with outline CSS property)	no border
language	What language is this text in, e.g. en-US (do we want this or do we use IA2::locale and make sure each locale change gets separate accessible object?)	same as doc itself?
misspelled	"true" if it is marked by the inline spell checker for being incorrectly spelled. Attribute not present in all other cases.	Not misspelled
static	"true" for list bullet/numbering text or layout-inserted text (such as via CSS pseudo styles :before or :after)	false
z-index	a number which indicates how low/high an element is	0 (normal height)

Linux Foundation Proposed (ATK + IA2)

Note: the following characters in names and values need to be escaped with a backslash: backslash, colon, comma, equals, and semicolon.

Attribute name	Attribute description	Default value if attribute not exposed
background-color	Background color as u, u, u	no default, always specified?
color	Foreground color as u, u, u	no default, always specified?
font-family	The computed font name	no default, always specified?
font-size	Font size in pixels	no default, always specified?
font-style	italic (oblique not currently supported)	normal
font-weight	The computed font weight (100, 200, 300, 400, 500, 600, 700, 800, 900; normal = 400, bold = 700)	400
text-line-through-style	solid (only exposed if if there is a line through)	no line through
text-underline-style	solid (only exposed if if there is an underline)	no underline
line-height	CSS2 (Too vague? Wouldn't it be better to only provide one unit ever?)	no default, always specified?
margin-bottom	CSS2 (Too vague? Wouldn't it be better to only provide one unit ever?)	no default, always specified?
margin-left	CSS2 (Too vague? Wouldn't it be better to only provide one unit ever?)	no default, always specified?
margin-right	CSS2 (Too vague? Wouldn't it be better to only provide one unit ever?)	no default, always specified?
text-align	left/center/right/justify	left
text-indent	CSS2 (Too vague? Wouldn't it be better to only provide one unit ever?)	no indent
text-position (for sup/sub)	?	?

Currently unsupported

The following text attributes, which are in the IAccessible2 text attribute specification, are not currently supported due to today's limitations of text formatting on the web:

• text-line-through-mode
• text-line-through-type
• text-underline-type
• text-underline-width
• text-underline-mode
• list -- the list bullet text is exposed via the accessible text interface, with the text attribute static=true
• tab-stop
• text-shadow

References

• IA2 Text Attributes - IAccessible2 Text Attributes Documentation