Algorithms like Brotli compression and Zstandard compression achieve even greater efficiency by allowing the use of dictionaries of commonly encountered strings, so you don't need any copies of them in the compressed resource. These algorithms ship with a predefined default dictionary that is used when compressing HTTP responses.
Compression Dictionary Transport builds on this by enabling you to provide your own dictionary which is especially applicable to a particular set of resources. The compression algorithm can then reference it as a source of bytes when compressing and decompressing the resource.
Assuming the references from the previous example are included in that common dictionary, this could be further reduced to this:
[d0:9]a[d10:20]Hello World![d42:46] [d0:9]b[d10:20]I am here[d42:46]
The dictionary can either be a separate resource that is only required for Compression Dictionary Transport, or it can be a resource that the website needs anyway.
For example, suppose your website uses a JavaScript library. You would typically load a specific version of the library, and might include the version name in the name of the library, like