Rationale
There is no replacement for clear and consistent navigation, but providing a sitemap can be a useful addition to your site, particularly on large and complex sites. It helps the user to understand the structure of the site, to orient themselves within the site, and to find the page or information they want, quickly. If it is not in text format, however, it may not be accessible to all users.
Techniques
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Be concise and don't over complicate the sitemap. It doesn't need to list each and every page of your site, particularly if your site is large. Sitemaps should provide more information than can be gained from the primary navigation, but not so much that it causes information overload. A happy medium would be to show the primary and secondary navigation.
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Headings and nested lists can be used to create clear, easy to understand text-based site maps. Headings correctly marked up using H1 to H6, should be used to lay out the sitemap in a way that reflects the structure of your site. This makes the page easier to scan visually and provides an additional means for a screen reader user to navigate in a page as keyboard functions allow users to jump between headings. Nested list can then be used to group related links under headings.
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Text format should be used to ensure it is accessible to all users. The benefit of having a clear overview of the site structure is lost if the user has to work to find the information or can not access it because it relies on JavaScript or Flash.
Testing tips
Check that a sitemap is present in the site and that it is navigable and logical.
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Browser - Check the main navigation for links to a "Sitemap". Check the sitemap page itself for correctly marked up headings, paragraphs and list.
Further information
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Headings information in the structure section.
Compliance with WCAG 1.0
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13.3 Provide information about the general layout of a site (eg a site map or table of contents). [Priority 2] In describing site layout, highlight and explain available accessibility features.
For more information on techniques visit the Web Accessibility Initiative techniques page.