• Your PDF Remembers What You Deleted

  A PDF does not have a current version. It has a stack of them, and the one your reader shows you is whichever one is on top. That is not a bug, but a clause in the specification, it is the reason PDFs can be signed and annotated without …

• Looping Is Not in the GIF Spec

  The GIF89a specification defines the signature, the color tables, the LZW encoder, the four interlacing passes, transparency, and the per-frame delay. It does not define animation looping. Download it and search: the word “loop” does not …

• JPEG Quality 80 Is Not a Setting

  MP4’s boxes are exact about everything: every byte accounted for, every offset written down. JPEG is exact about its structure too, and then hands you one control that has no defined meaning at all. It destroys things. That is the entire …

• An AVIF Is an MP4 With One Frame

  XML’s answer to structure was a schema language, a query language, and a namespace system. MP4’s answer is eight bytes, and it turned out to be enough to absorb an entire industry. Every byte in the file lives inside a box, and every box …

• Every XML Feature Is One You Turn Off

  Last post ended on a question. XML is the format in this series that specified everything, so what did all that specifying buy? An answer for every question, and an attack surface made entirely of answers. The other formats in this series …

• Norway Is Not a Boolean

  JSON’s problem is that its specification is too small. It tells you 9007199254740993 is a well-formed number and then declines to say which number. YAML went the other way. The 1.2.2 specification is a book. It has a formal grammar, a …

• Your JSON Parser Disagrees With Mine

  Markdown has too many specs. CSV has one that nobody agreed to follow. JSON is supposed to be the happy ending: a grammar small enough to print on a business card, standardized twice over, and elevated to a full Internet Standard. It is, …

• Nobody Agrees What a CSV Is

  CSV is simple but powerful. Values, separated by commas. It’s easy to understand and use. It is also, by a wide margin, the one that destroys the most data. That’s not a paradox. It’s cause and effect. A format simple enough that everyone …

• Word Documents Used to Be Filesystems

  Last post ended on a promise: a .docx is a ZIP file, and you already know how ZIP works. That’s true, and it’s the smaller half of the story. The interesting part is what .docx replaced, because the old .doc format was doing something …

• ZIP Files Are Read Backwards

  Every format in this series so far reads front to back. PNG starts with a signature and you walk chunks in order. A text file is bytes from the beginning. Markdown parsers scan line by line, top to bottom. ZIP reads backwards. The index is …