• Structured documents are important as they allow computers to work with the data.
• SGML was created as a means to bridge markup between working groups.
• Professional typesetting systems once cost 50k$-100k$ USD.
• People made GUI tools to typeset SGML documents and edit DTDs.
• Railroad diagram: a way to visualize parsers.
• Format tagging: when text has style information defined explicitly in the tag.
• Structure tagging: when structural information is in the name of the tag, not implied. ex

  ,

  , instead of

  .

• Content tagging: when tags describe what kind of content is happening.
• A DTD is very important for defining how a particular document is to be read.
• When making a document system; interview users and make their feedback part of the design process. Don't design useless shit nobody uses.
• Parsers have "capabilities" and "features" to identify things like the maximum size of a tag, whether including other documents is allowed.

I gave up around page 252. It goes in to depth on how tags work and are defined. But it's the kind of thing that probably deserves to be in a hypertext reference and not a Zettelkasten card.

DTDs

• ?: optional, zero or one
• +: required, one or more
• *: optional, zero or more
• ,: is followed by
• (): grouping
• |: alternatives
• &: must appear but order is unimportant

Entity

• Maps an entity name to some particular content.
• Apparently this can also import another file.
• Entities also exist so you can type characters that exist in the output typesetting system but not ASCII keyboards. This was also before Unicode.

Now when you see &onehalf; it SHOULD be replaced with 1/2.

Element

Defines elements and what is allowed to appear within them.

Attribute list

Defines what attributes are attached to tags and whether they are required or not.

Document type

Defines the layout of the document.

SGML declaration

Holds "bootstrap" information.

Body

• Start tags; ex
• End tags; ex
• Entity reference; ex &
• Comment; ex
• Processing instruction; ex
• Character reference; ex -
• Marked declarations; ex
• Short reference use; ex
• Link set use; ex
• Start tags may not be required if the grammar says they may be implied.
• End tags may not be required if the tag is marked as empty or the grammar says it is implied.