alt attribute and title attribute_CSS/HTML

作者：JunChen 2005-5-23 12:00:35
原文：http://www.456bereastreet.com/archive/200412/the_alt_and_title_attributes/
翻译：JunChen

首发：http://blog.handsbrain.com/junchen/archive/2005/05/21/3355.aspx
版权：本文版权归译者Junchen所有，转载请先联系译者。
当浏览器卖主扭曲了标准并且自顾自的不按规则去做一些事，他们可能会造成一些问题，或者至少产生了混淆。例子之一就是一些浏览器处理alt属性（一般会被错误的称作alt标签）的方式，比如拥有大量用户的Windows的IE浏览器。

替换文字（alt text）并不是用来做提示（tool tip），或者更加确切的说，它并不是为图片提供额外说明信息的。相反地，title属性才应该用来为元素提供额外说明信息。这些信息在大部分图像浏览器里显示为提示（tool tip），虽然制造商可以任意采取其他方式渲染title属性的文字。

很多人看来对这两个属性感到迷惑（最近这个问题在Web Standards Group邮件列表里变多了）， 所以我写下我的想法，如何去用它们。

alt属性

为不能显示图像、窗体或applets的用户代理（UA），alt属性用来指定替换文字。替换文字的语言由lang属性指定。
来源：How to specify alternate text.

Alt属性（注意是“属性”而不是“标签”）包括替换说明，对于图像和图像热点是必须的。它只能用在img、area和input元素中（包括applet元素）。对于input元素，alt属性意在用来替换提交按钮的图片。比如：.

使用alt属性是为了给那些不能看到你文档中图像的浏览者提供文字说明。这包括那些使用本来就不支持图像显示或者图像显示被关闭的浏览器的用户，视觉障碍的用户和使用屏幕阅读器的用户。替换文字是用来替代图像而不是提供额外说明文字的。

在写替换文字前仔细想想，保证那些文字确实为那些看不到图像的人提供了说明信息，并且在上下文中有意义。对于那些装饰性的图片可以使用空的值（alt=""，引号中间没有空格），而不是使用不相关的替换文字比如“blue bullet”或者“spacer.gif”。不要忽略它，如果你忽略了，那么一些屏幕阅读器会直接阅读图像文件的文件名，那些文字浏览器，比如Lynx会显示图像文件的文件名，而那对于你的浏览者就没什么用了。

包含文字的图像图片设置替换文字是最简单的，图像中包含的文字一般来说就可以作为alt属性值。

至于替换文字的长度，看看WCAG 2.0（网站内容可用性指南2.0）是怎么说的：

Alt属性值得长度必须少于100个英文字符或者用户必须保证替换文字尽可能的短。
我把它理解为“尽可能短，尽需要长”。

即使你想让它显示为提示（tool tip），也不要给文字元素使用alt属性，这并不是它的用法。至今据我所知，那样做仅能在Windows的IE浏览器和古老的Netscape 4.*（windows版本）有效。没有一个Mac的浏览器会将它显示为提示（tool tip）。

当浏览器把替换文字显示为提示（tool tip）后，那些错误使用alt属性的行为也受到了鼓励。一些人开始写无意思的替换文字，因为他们趋向于认为它是一个额外的说明信息，而不是不能显示图像的替换。其他人可能不想让提示（tool tip）出现，然后就完全忽略了写alt属性值。这些错误的做法，都给那些不能看到图像的浏览者造成了困难。

额外的说明信息和非本质的信息请使用title属性。

title属性

title属性为设置该属性的元素提供建议性的信息。
来源: The title attribute.

title属性可以用在除了base，basefont，head，html，meta，param，script和title之外的所有标签。但是并不是必须的。可能这正是为什么很多人不明白何时使用它。

使用title属性提供非本质的额外信息。大部分的可视化浏览器在鼠标悬浮在特定元素上时显示title文字为提示信息（tool tip），然而这又由制造商来决定如何渲染title文字。一些浏览器会将title文字显示在状态栏里。比如早期版本的Safari浏览器。

A good use of the title attribute is to add descriptive text to a link, especially when the purpose of the link is not very clear from the link itself. This way visitors know where the links will take them, and they won't load a page that they may not be interested in at all. Another potential application is to provide additional descriptive information for images, such as dates or other non-essential information.

The title attribute value can be set longer than the alt attribute value. Be aware, however, that some browsers will truncate text that is too long (such as tooltips or other). For example, Mozilla's core browser can only display the first 60 characters. This is considered a Mozilla bug and is something you should be aware of.

Think before using

My advice is to keep your alt text to the essence. In most applications, it should be left blank, alt="" (note that there are no spaces between the quotation marks). Think about those images, what information do they provide to those who are viewing them, what words should you use to describe it, or what information should you provide to people who cannot see the images? Will it really help someone who can't see the image to write the alt text as "Photo: CEO standing outside a building, wearing a gray suit and black tie, looking at the sky"? If you think so, then write it. In many cases, I think it's better to leave the replacement text blank.

For the title attribute, it is difficult to give strict usage instructions. I mostly use it on links that are not self-explanatory, such as the same link text on the same page, but different linked pages. Sometimes more descriptive text is provided for some buttons or form elements.

Longer description

When an image requires a longer description than the alt attribute allows, there are some options.

The longdesc attribute can be used to provide a link to a separate page containing a text description of the image. This means linking the viewer to another page, which may cause difficulty in understanding. In addition, browser support for the longdesc attribute is inconsistent and not very good.

The longdesc attribute can contain a link to other parts of the current document (anchor) instead of linking to another page. In the Accessibility footnotes, Andy Clarke explains very well how to apply it.

Description links (D links) can be used to supplement longdesc. A description link is a regular link to a page containing alt text. The link is placed next to the image and is available in all browsers. There are many different opinions on its effectiveness, and I personally don't like this note. WCAG too, in their working draft HTML Techniques for WCAG 2.0, description links are "deprecated".

If a long description of the image is useful to any viewer, then you should consider simply displaying it in the same document, rather than linking to other pages or hiding it. So everyone can read it. This is a simple, low-tech approach.

More information

Want to know more about alt, title and longdesc attributes? Look here:

Guidelines on alt texts in img elements
Writing good ALT text
Accessible alternatives
Accessibility footnotes
The image problem
Title Attribute – Your Take
Using link Titles to Help Users Predict Where They Are Going
How to specify alternate text
The title attribute