In-place content substitution via code-invoking link

Content including links to behaviors (code which can be executed and return supplemental content for insertion, or can modify existing content) is stored, and at run-time, the links to behaviors are followed and the supplemental content or the modifications to existing comment are used to create a final version of content which will be presented to the user. Security enhancements including a security check ensure that only behaviors which are secure will be run. Default content may be provided, which may be inserted if the security check is not passed, or if the content returned from the behavior is unusable for some reason.