[...] Since neither of the two authoring formats defined
in this specification are applications of SGML, a
validating SGML system cannot constitute a conformance
checker [...].
<title>Tag omission in HTML paragraphs</title>
<p>This is the first paragraph.
<p>This is the second.
How is it parsed by SGML?
A minimal DTD for HTML
Tag omission indicators
<!ENTITY % metadata "title|script">
<!ENTITY % scripting "script|template">
<!ELEMENT html O O (head,body) +(%scripting;)>
<!ELEMENT head O O (%metadata;)*>
<!ELEMENT body O O ANY>
<!ELEMENT p - O ANY -(p)>
O O (double capital letter O for "omission")
the html, head, and body elements admit
start- and end-tag omission
A minimal DTD for HTML
Tag omission indicators
<!ENTITY % metadata "title|script">
<!ENTITY % scripting "script|template">
<!ELEMENT html O O (head,body) +(%scripting;)>
<!ELEMENT head O O (%metadata;)*>
<!ELEMENT body O O ANY>
<!ELEMENT p - O ANY -(p)>
O -
the p element admits end-tag omission only
A minimal DTD for HTML
Content exceptions
<!ENTITY % metadata "title|script">
<!ENTITY % scripting "script|template">
<!ELEMENT html O O (head,body) +(%scripting)>
<!ELEMENT head O O (%metadata;)*>
<!ELEMENT body O O ANY>
<!ELEMENT p - O ANY -(p)>
ANY -(p)
p admits any element except panywhere as
content
Parsing HTML
<!DOCTYPE html SYSTEM "html-minimal.dtd">
<title>Tag omission in HTML paragraphs</title>
<p>This is the first paragraph.
<p>This is the second.
Parsing HTML
<!DOCTYPE html SYSTEM "html-minimal.dtd">
<html>
<title>Tag omission in HTML paragraphs</title>
<p>This is the first paragraph.
<p>This is the second.
SGML creates an html element if it isn't there, knowing that an
html document element must be the first content element in
an HTML file
Parsing HTML
<!DOCTYPE html SYSTEM "html-minimal.dtd">
<html>
<head>
<title>Tag omission in HTML paragraphs</title>
<p>This is the first paragraph.
<p>This is the second.
SGML infers the head element if it isn't there, since the
content model requires it at the start of html's content
Parsing HTML
<!DOCTYPE html SYSTEM "html-minimal.dtd">
<html>
<head>
<title>Tag omission in HTML paragraphs</title>
<p>This is the first paragraph.
<p>This is the second.
SGML accepts the title element as child content of head,
as allowed by heads model group expression
Parsing HTML
<!DOCTYPE html SYSTEM "html-minimal.dtd">
<html>
<head>
<title>Tag omission in HTML paragraphs</title>
</head>
<p>This is the first paragraph.
<p>This is the second.
SGML infers the end-element tag for the head element
since the p element following title isn't allowed to
occur in head
Parsing HTML
<!DOCTYPE html SYSTEM "html-minimal.dtd">
<html>
<head>
<title>Tag omission in HTML paragraphs</title>
</head>
<body>
<p>This is the first paragraph.
<p>This is the second.
SGML infers the body element if it isn't there, since it's
required to follow the head element
Parsing HTML
<!DOCTYPE html SYSTEM "html-minimal.dtd">
<html>
<head>
<title>Tag omission in HTML paragraphs</title>
</head>
<body>
<p>This is the first paragraph.
<p>This is the second.
SGML accepts the first p element as content of body
Parsing HTML
<html>
<head>
<title>Tag omission in HTML paragraphs</title>
</head>
<body>
<p>This is the first paragraph.</p>
<p>This is the second.
</body>
</html>
SGML infers the end-element tag for p, since p isn't
accepted as content of p
Parsing HTML
<html>
<head>
<title>Tag omission in HTML paragraphs</title>
</head>
<body>
<p>This is the first paragraph.</p>
<p>This is the second.</p>
</body>
</html>
SGML infers the end-element tags for p, body, and html at the
end of the document
Attribute minimization
Given the following attribute declaration
<!ATTLIST option selected (selected) #IMPLIED>
these element/attribute specifications are equivalent:
A p element's end tag may be omitted if the p element is
immediately followed by an address, article, aside, blockquote, details,
div, dl, fieldset, figcaption, figure, footer, form, h1, h2, h3, h4, h5,
h6, header, hr, main, menu, nav, ol, p, pre, section, table, or ul,
element, or if there is no more content in the parent element and the
parent element is an HTML element that is not an a, audio, del, ins, map,
noscript, or video element.
Content model for P
Content: Phrasing content.
is translated to this element declaration
<!ELEMENT p (#PCDATA|%phrasing;)*>
Note Text (character data content) is also phrasing content.
Tag omission rules for P
A p element's end tag may be omitted if the p element is
immediately followed by an address, article, aside, blockquote, details,
div, dl, fieldset, figcaption, figure, footer, form, h1, h2, h3, h4, h5,
h6, header, hr, main, menu, nav, ol, p, pre, section, table, or ul,
element, or if there is no more content in the parent element and the
parent element is an HTML element that is not an a, audio, del, ins, map,
noscript, or video element.
Tag omission rules for P
A p element's end tag may be omitted if the p element is
immediately followed by an address, article, aside, blockquote, details,
div, dl, fieldset, figcaption, figure, footer, form, h1, h2, h3, h4, h5,
h6, header, hr, main, menu, nav, ol, p, pre, section, table, or ul,element, or if there is no more content in the parent element and the
parent element is an HTML element that is not an a, audio, del, ins, map,
noscript, or video element.
Complete declaration for P
Complete DTD declarations for the p element
<!ENTITY % phrasing "a|abbr|area|...">
<!ENTITY % block "address|article|...">
<!ELEMENT p - O
(#PCDATA|%phrasing;)* -(%block;)>
Complete DTD
<!-- The html element (section 4.1).
Content: A head element followed by a body element.
Tag omission: An html element's start tag can be omitted if the first thing
inside the html element is not a comment. An html element's end tag can be
omitted if the html element is not immediately followed by a comment. -->
<!ELEMENT html O O (head,body) +(script)>
<!ATTLIST html
%extensionattrs;
accesskey NMTOKENS #IMPLIED
class NMTOKENS #IMPLIED
contenteditable (true|false) #IMPLIED
contextmenu IDREF #IMPLIED
$ cat test.html
<!DOCTYPE html SYSTEM "html51.dtd">
<title>Test</title>
<body>
<ol hidden reverse>
<li>One</li>
<li>Two</li> </ol>
$ sgmlproc test.html
"testfile.html": line 4: fatal: 'reverse':
token not in token group for any attributes
$ head -1 test3.html
<!DOCTYPE html SYSTEM "html51e.dtd">
$ grep 'ELEMENT datalist' html51e.dtd
<!ELEMENT datalist - -
((#PCDATA|%phrasing;)*|(option|%scripting;)*)
-(%flow_only;)>
$ osgmlnorm test3.html
content model is ambiguous:
when no tokens have been matched, both the 1st
and 2nd occurrences of "TEMPLATE" are possible
tells SGML that hr
elements don't have end-element tags
Section wrappers
reveal.js expects slides in section elements:
<html>
<body>
<section>
<h1 id="first-slide">Heading for first slide</h1>
</section>
<section>
<h1 id="second-slide">Heading for second slide</h1>
</section>
...
</body>
</html>
So we need to wrap portions between hr elements
into section wrappers
Section wrappers
<!ELEMENT body - - (section,(hr,section)*)>
makes SGML generate an initial section element before any child element of body
<!ELEMENT section O O ANY -(hr)>
causes SGML to end section elements and open
new ones on hr elements, since hr elements aren't
allowed in section content by the -(hr) exclusion
exception
Section wrappers
# Heading for first slide #
---
# Heading for second slide #
Parsing this with the two declarations yields
<body>
<section>
<h1>Heading for first slide</h1>
</section>
<hr>
<section>
<h1>Heading for second slide</h1>
</section>
</body>
Section wrappers
<h1>Heading for first slide</h1>
<hr>
<h1>Heading for second slide</h1>
Equivalently, parsing this HTML fragment yields
<body>
<section>
<h1>Heading for first slide</h1>
</section>
<hr>
<section>
<h1>Heading for second slide</h1>
</section>
</body>
Removing hr elements
<body>
<section>
<h1>Heading for first slide</h1>
</section>
<hr>
<section>
<h1>Heading for second slide</h1>
</section>
</body>
So this will also produce hr elements
which isn't desired; we're
going to remove hr elements now
Removing hr elements
<!DOCTYPE html [
...
]>
<!LINKTYPE tohtml html html [
<!LINK #INITIAL
html html
body body
section section>
]>
In SGML, we can do so by declaring a link process
filtering/transforming source markup into result markup
Removing hr elements
<!DOCTYPE html [
...
]>
<!LINKTYPE tohtml html html [
<!LINK #INITIAL
html html
body body
section section>
]>
The tohtml link process is declared to
transform html to html markup, in turn
Removing hr elements
<!DOCTYPE html [
...
]>
<!LINKTYPE tohtml html html [
<!LINK #INITIAL
html html
body body
section section>
]>
The #INITIAL (and only) link set
copies the specified elements to its result;
hr elements, however, won't get copied
slides.sgml
<!DOCTYPE html [
<!ENTITY slides SYSTEM "demo-slides.md">
<!ENTITY % md_shortref_maps ...> ...
<!ELEMENT html - - (head?,body)>
<!ELEMENT body - - (section,(hr,section)*)>
<!ELEMENT section O O ANY -(hr)>
<!ELEMENT hr - O EMPTY>
]>
<!LINKTYPE tohtml html html [
<!LINK #INITIAL html html body body section section>
]>
<html><body>
&slides
</body></html>
Fixing invalid result markup
Now that hr won't get copied,
bodys content model is violated
(which requires section elements
to be separated by hr elements):
(section,(hr,section)*)
We therefore declare an additional DOCTYPE;
we'll keep html as name of the result DOCTYPE
that doesn't require hr elements and use slides
as source DOCTYPE name for parsing
slides.sgml
<!DOCTYPE slides [
<!ELEMENT slides - - (head?,body)>
<!ELEMENT body - - (section,(hr,section)*)>
<!ELEMENT section O O ANY -(hr)>
<!ELEMENT hr - O EMPTY>
<!ENTITY slides SYSTEM ...> ...
]>
<!DOCTYPE html [
<!ELEMENT html - - (head?,body)>
<!ELEMENT body - - (section)*>
<!ELEMENT section - - ANY>
]>
<!LINKTYPE tohtml slides html [
<!LINK #INITIAL slides html body body section section>
]>
<slides><body>
&slides
</body></slides>
Fixing invalid result markup
The slides DOCTYPE contains all the declarations of html;
the document element is renamed to slides, however
The html DOCTYPE contains all declarations as before
except for the changed body content model (and
for the unnecessary markdown SHORTREF declaration)
The copy template
<!LINK #INITIAL html html body body section section>
this link set will only copy the explicitly
handled/mapped source element and text content;
it won't copy other source markup elements
So our result markup will just contain the plain text
content of our slides which isn't what we want
The copy template
We could change the link rule to map each
individual HTML element:
<!LINK #INITIAL
...
a a
b b
div div
...
-- etc. -- >
But this would require us to declare and copy all elements
over to result markup individually.
The copy template
<!DOCTYPE #IMPLIED SYSTEM [
<!ENTITY content SYSTEM "<osfd>0">
]>
<section>
&content;
</section>
The "copy" template, stored in a separate copy.sgml file,
looks like this
<osfd>0 is a formal system identifier
representing the Unix standard input file number 0 (stddin)
The copy template
sgmlproc copy.sgml < input.txt
if used on the command line directly like this,
SGML receives the content of the input.txt file
as standard input when invoked using sgmlproc
For this to work, a section.dtd file must be
present in the current working directory; SGML will
attempt to resolve a DTD file inferred from the
name of the first encountered content
element (due to <!DOCTYPE SYSTEM #IMPLIED>)
The copy template
On the other hand, if used from a master, a template's standard
input resolves to the child content
of the element on which it is applied in the master
<section>
<!- a template will receive this as
standard input when applied on the
section element: -->
<h2>Section heading</h2>
<p>Section body content</p>
</section>
The copy template
Moreover, when used from a master, SGML resolves
DOCTYPE #IMPLIED SYSTEM as if a section.dtd
file were present in the current directory, containing
all the target markup declarations from the calling processing
context:
<!ELEMENT html - - (head?,body)>
<!ELEMENT body - - (section)*>
<!ELEMENT section - - ANY>
Applying the copy template
To make SGML apply the copy template on the section
element, we use these link set declarations:
We re-declare the former section section link rule such that
it applies the copy template to section elements
Applying the copy template
<!NOTATION sgml
PUBLIC
"ISO 8879:1986//NOTATION Standard Generalized Markup Language (SGML)">
<!NOTATION copy SYSTEM>
<!ATTLIST #NOTATION copy superdcn name #FIXED sgml>
<!ATTLIST section template NOTATION (copy) #IMPLIED>
These declarations tell SGML that the value of the template
attribute carries an SGML file that must be processed special
Applying the copy template
<!NOTATION sgml ...
declares sgml as the name of a notation identified by
the "ISO ..." public identifier (sgmljs.net SGML recognizes the
public identifier, but sgml, on the other hand, could be any
other identifier here as long as it is used consistently with
other declarations)
Applying the copy template
<!NOTATION copy SYSTEM>
declares the copy file (without .sgml suffix) as
a notation
declares the templatedata attribute (attribute of
a notation) as having NOTATION attribute type, with copy
the only allowed value (and, by using #IMPLIED, as an
optional link attribute of the template element)
Applying the copy template
<!ATTLIST #NOTATION copy superdcn name #FIXED sgml>
declares the distinguished superdcn data attribute
of copy as having the value sgml, which will make sgmljs.net
SGML treat the notation as SGML template
Applying the copy template
<!NOTATION sgml
PUBLIC
"ISO 8879:1986//NOTATION Standard Generalized Markup Language (SGML)">
<!NOTATION copy SYSTEM>
<!ATTLIST #NOTATION copy superdcn name #FIXED sgml>
<!ATTLIST section template NOTATION (copy) #IMPLIED>
<!LINK #INITIAL
slides html
body body
section [ template=copy ] section>
Invoking slide preparation
sgmlproc slide.sgm > slides.html
we can now invoke sgmlproc to preview the generated HTML
on the command line like this (provided we have the slides
and copy files in place in the same directory)
sgmljs.net SGML will now replace every section element by
the expanded content of the copy template, with the child
content of the respective section elements as value of
the content entity
More on templates
<!-- template -->
<!DOCTYPE #IMPLIED SYSTEM [
<!ENTITY param SYSTEM>
]>
<div>¶m;</div
<!-- applying the template --->
<!LINK div [ param="value" ] div>
github.com/whatwg/html/commit/6e305c457e42276bf275b8432302a32c929b0eb8
