Alt text for images
One solution is to put the InChI as content of the @alt attribute of the HTML <img> element. This has the downside that it has no explicit semantic meaning. For example, the Molecule Of The Day blog is using this approach. It's an excellent start, but not the solution.
Another option is to put it in as keyword, in the HTML <head> element: <meta name="keywords" content="InChI=1/CH4/h1H4"/>. But Google does not index this, so the use is restricted.
The most promosing alternative, however, is to put it in using the <span> element, in combination with microformats or RDFa, Like this: . It does not show up, does it? But it is really there, as you would see, if you have the special Greasemonkey script installed.
This is the HTML code for this example:
<span class="chem:inchi" style="font-size: 0%; visibility: hidden;">InChI=1/CH4/h1H4</span>
The @style attribute marks the text's visibility as hidden, and the font-size is set to 0%. It is important not to set it to zero itself, because many web browsers do not interpret zero font size correctly, and take the default font size instead.
This should solve the standing problem that we would like to include the InChI's in our blogs, if it would just not be so long and unreadable. Just hide it.
Update: Daniel informed me that Google won't index text marked 'visibility: hidden' and may even mark your webpage as spam :( Not the solution either. Read the comments for more thoughts.