If you want to include in your page source things you want to dynamically display, such as error messages or tooltips or confirmation messages – without having them show up in Google† – consider using the `<script type="text/html"> ... HTML ... </script>` trick. Then you can grab the content via e.g. `.innerHTML` and place it in your lightbox or use it for templating, etc. See also: ICanHaz.js.

For the opposite – showing text to search engines and screen readers, but not visual browsers' end-users – position the content off-screen. http://webaim.org/techniques/css/invisiblecontent/#technique...

† I haven’t explicitly tested that search engines ignore contents of script tags w.r.t. indexing, but would be extremely surprised if the assumption was shown to be false.