If the image is relevant to the content of the document, you must supply alternate text that will appear in text-only browsers, and that will appear in other browsers when an image is loading or when image loading is disabled.
如果图像和文档相关联，那么您应为图像提供替代文本，可以供仅能显示文字的浏览器使用，也可以供其他浏览器在加载图像或禁用图像时显示。