D ጷ and H ጷ?
[I] and [B] links?
This can happen when the required fonts are not available. Make sure that your browser's page encoding is set to Unicode (UTF-8), and that you have a suitable Unicode font (you can find some here). However, the Unicode specification receives regular updates, and it is likely that many characters in the latest version are not yet supported by existing fonts.
As for the question marks / boxes themselves, the most popular browsers have slightly different ways of dealing with these missing characters. Internet Explorer and Firefox 2 show question marks, Opera shows small rounded boxes, and Firefox 3+ will show small boxes with the hex code of the missing character. Using the Arial Unicode MS font may cause a question mark enclosed in a diamond to appear instead.
This is a rendering bug with combining / non-spacing characters :(
These characters should show up in recent browsers such as Opera 9.5 and Firefox 3.
D and H ?These are the Decimal and Hexadecimal HTML Entities for each character. Even if you don't have an HTML editor which supports Unicode, you can still insert Unicode characters into your pages by pasting either of these codes. Decimal is easier on the brain, but the Unicode Consortium uses hexadecimal almost exclusively.
[I] and [B] links?These mean "Isolate" and "Browse" respectively, and they may only show up in certain modes. "Isolate" shows a 4x larger preview of a character with some extra information, and "Browse" will allow you to return to browsing the Unicode database, beginning from that character.
Character descriptions in Unicode 1.0 vary from subsequent specifications. As a way of indentifying which search results have been found via the old and new descriptions, these colours are used. Although you can search both new and old descriptions, you may get a mixture of results when using the quick links listbox or searching all descriptions.
There are gaps in the Unicode range where a character doesn't exist (yet), so the script simply doesn't show a character box for that ID. Searches are limited to show only the first results, so be sure to use concise terms. It's also possible that an existing character has been deliberately omitted by the URL, as this is a feature of the browse and range modes. The bolded part of this example URL specifies the characters with the decimal IDs of 9250 and 9251 to be omitted: ?s=9216&pp=37&o=9250,9251
These are control characters; they have no practical use in webdesign, and will in fact cause this web application to fail W3C validation. When trying to view these characters, you may either be "silently redirected" or shown an error. If you really must see what they look like, here are some aliases.
If a hypen-separated range ends with a lower number than the beginning, or if the number either side of the hyphen is missing, the query cannot be performed. Check the query and try again. There is also a maximum amount of results which a range query can return.
Wazu Japan - Tons of fonts categorised by writing system
Code2000 - Get Code2001 and Code2002 as well to fill most of the gaps
Alan Wood's Unicode Resources - A compendium of fonts for various scripts
MPH 2B Damase - Supports many scripts in the Supplementary Multilingual Plane
Unicode Fonts for Ancient Scripts - Akkadian = Cuneiform = MIND BLOWN
Arial Unicode MS also covers a lot of the Unicode range, but I can't link that because I don't want to be arseraped with legalese