3.1 General

A subtitle document shall not conform to the Text and Image Profiles simultaneously.

Where a subtitle document conforming to the Image Profile is used in an application there is often a requirement for the same subtitle/caption content to be simultaneously available in text form. The Success Criteria for Guideline 1.1 of the Web Content Accessibility Guidelines 2.0 (see [WCAG20 Guideline 1.1] require that an implementation provides text alternatives for all non-text content. This text alternative is intended primarily to support users of the subtitles who cannot see images. For these applications, two distinct subtitle documents, one conforming to the Text Profile and the other conforming to the Image Profile, should be offered – or a text equivalent string may be provided in an alt attribute for each image in the Image Profile subtitle document. 

The WCAG20 guidelines for "Text Alternatives" (see [WCAG20 Conformance] state that accessible text alternatives must be "programmatically determinable." This means that the text must be able to be read and used by the assistive technologies (and the accessibility features in browsers) that people with disabilities use. In the context of IMSC, where the images of an Image Profile subtitle document usually represent subtitle or caption text, the guidelines for authoring text equivalent strings given in the html5 specification are appropriate: (see [HTML5 Images of text]).

Thus, for each subtitle in the Image Profile subtitle document, the text equivalent content in the Text Profile subtitle document should be written so that it conveys all essential content and fulfils the same function as the corresponding subtitle image. In the context of subtitling and captioning, this content will be (as a minimum) the verbatim equivalent of the image without précis or summarisation. However, the author may consider including extra information to the text equivalent string in cases where styling is applied to the text image with a deliberate connotation, as a *functional* replacement for the applied style. E.g. In subtitling and captioning, italics may be used to indicate an off screen speaker context (for example a voice from a radio). An author may choose to include this functional information in the text equivalent; for example, by including the word "Radio: " before the image equivalent text. It should also be noted that images in an Image Profile subtitle document that are intended for use as *captions*, i.e. intended for a hard of hearing audience, may already include this functional information in the rendered text.

In addition applications of the IMSC document profiles that offer two distinct subtitle documents must also make it possible for people using assistive technologies to find these text alternatives when they encounter non-text content that they cannot use (see [WCAG20 Conformance]. To accomplish this, the WCAG20 guidelines say that the text must be "programmatically associated" with the non-text content. This means that the user must be able to use their assistive technology to find the alternative text (that they can use) when they land on the non-text content (that they can't use).


6.4 alt attribute

The alt attribute is defined to allow an author to provide a text string equivalent for an image. This text equivalent may be used to support indexation of the content and also facilitate quality checking of the document during authoring. If a smpte:backgroundImage attribute is applied to a div element, an imsc1:alt attribute should be applied to the div element and should contain a string value that is a verbatim text  equivalent of the image source referenced by the smpte:backgroundImage. 

It should be noted that *in contrast to the common use of alt attributes within html content*, the imsc1:alt attribute content is not intended to be displayed in place of the image if the image file is not (able to be) loaded. The imsc1:alt attribute content may however be read and used by the assistive technologies.

WCAG20 Guideline 1.1	http://www.w3.org/TR/2006/WD-WCAG20-20060427/guidelines.html#text-equiv

HTML5 Images of text	 section 4.7.1.1.5 Images of text of the draft document for HTML5 - A vocabulary and associated APIs for HTML and XHTML (W3C Last Call Working Draft 17 June 2014). : http://www.w3.org/TR/html5/

WCAG20 Conformance
http://www.w3.org/TR/UNDERSTANDING-WCAG20/conformance.html#uc-text-alternatives-head