Validation Bookmarklets
These bookmarklets let you check pages for correctness, standards-compliance, and several aspects of accessibility.
(To keep bookmarklets in order to use them on other web pages, drag them to your Bookmarks Toolbar. Or, install them all at once.)
Browsers: 
is Internet Explorer for Windows, 
is Netscape 7 or Mozilla, 
is Netscape 4, and 
is Opera.
Details
Validates HTML using the W3C HTML validator.
On Windows and Linux, this bookmarklet even works for local files! (Supporting local HTML files was suggested by Stephan Niemz.)
Validates HTML, avoiding "missing doctype" and "missing charset" errors.
When the document does not contain a doctype, validates as HTML 4.01 Transitional. The W3C validator will say that the page is "tentatively valid" if you validate against a doctype not declared on a page, so using this bookmarklet won't cause you to forget to include a doctype.
Always uses the character set the browser used to interpret the page. The validator does not warn about character set overrides.
Tells you what type of server a site uses and displays an uptime graph.
Uses Web Sniffer to display HTTP headers.
Applies a "black and white TV" filter to the page.
Converts all colors on the page (including images) to gray scale. This will give you a rough idea of how your page looks to color-blind visitors. See the Web Content Accessibility Guidelines for color for some common problems and examples of how to provide redundant representations.
On a color table you'll notice that yellow is considered slightly brighter than cyan, which is considered slightly brighter than magenta. I don't know whether this is accurate.
(From Steve Kangas's accessibility bookmarklets.)
Lets you see what a page might look like on a PDA.
Written by Daniel Glazman.
Linearizes the text of the page and removes most non-text elements.
Lists the URLs of broken images.
Suggested by "Tomyellow" in #mozillazine.
Replaces each image with its alternate text.
This lets you quickly see how a page might look with images turned off. It should also give you rough idea of how a page would look to a blind user or someone using a console (text-only) browser such as Links. For more information about providing text equivalents, see the Web Content Accessibility Guidelines for alternate text.
This bookmarklet only replaces <img> tags; it doesn't do anything to image buttons, background images, or image maps.
Shows which images have missing and empty alts using borders and tooltips.
Key to borders:
- missing: dotted red
- empty: dashed gray
- present: solid green
Also sets image tooltips to show alt text and displays counts of (missing, empty, present) alt texts in the status bar.
Amazon.com is a good test page.
Lists images and their alternate text (or "missing" or "empty").
The idea for this bookmarklet came from a similar bookmarklet, "Get ALT Info" from scriptygoddess.
Validates HTML fragments in textareas.
Validates HTML fragments in textareas.
Validates XHTML fragments in textareas.
Validates XHTML fragments in textareas.
Quickly checks the well-formedness of XML fragments in textareas.
This bookmarklet is faster than the other "blogidate" bookmarklets because it does not open a new window and does not contact any servers. It only checks for well-formedness, which is necessary but not sufficient for validity. It works with XHTML (which is a type of XML) but not with HTML (which is not XML).
© 2000-2007 Jesse Ruderman (jruderman@gmail.com). Feedback is welcome.
Last modified November 1, 2006.