For small one-off test cases like these, this looks like a major improvement (28%) but in practice, the network overhead will dwarf the transmission time for 25 bytes and so a reduction to 18 bytes will be neglible. Hell, the HTTP headers alone will be much larger.
If the data set is fairly large, then the markup will contribute much less to the overall size, so the improvement will probably be more like 5-10%. After HTTP compression, the difference might be negligible.
Testing should definitely be done to see if the space saving is worth the additional parsing overhead (relaxing constraints makes the language more complicated).
Same applies to the original post, especially since the examples seem even more contrived. E.g., the author goes from <p></p> in (X)HTML to delimit a paragraph to {"format":"Paragraph","content":[]} in JSON, when one could just as easily contrive <format type="paragraph"></format> in XML versus {"p":[]} to make JSON look better.
If the data set is fairly large, then the markup will contribute much less to the overall size, so the improvement will probably be more like 5-10%. After HTTP compression, the difference might be negligible.
Testing should definitely be done to see if the space saving is worth the additional parsing overhead (relaxing constraints makes the language more complicated).
Same applies to the original post, especially since the examples seem even more contrived. E.g., the author goes from <p></p> in (X)HTML to delimit a paragraph to {"format":"Paragraph","content":[]} in JSON, when one could just as easily contrive <format type="paragraph"></format> in XML versus {"p":[]} to make JSON look better.