The document says what its marks stand for

Summary

Somebody who cannot see the page listens to it. The software reading it out has to know what is a heading, what is a paragraph, what is a table, which cell sits under which column heading, and above all in what order to say it all. A page that records only where each letter is drawn says none of that: the reading software then works through it in the order the text was drawn, which is rarely the order a person would read it.

The answer is a structure tree written alongside the content of the page: this group of words is a title, that one a paragraph, this block is a table and here is its heading row. The drawing does not change. What changes is that the file now knows what it drew — which also lets software pull the figures back out, and lets a search engine index the text properly.

A table too long for one page is cut, and carries on at the top of the next one. Its row of column headings is drawn again up there, so that somebody reading the second page still knows what each column holds.

No published rule says what that redrawn row is. The engine ties it to the same heading cell: the column headed March stays one column, its words stay selectable and read on both pages, and the structure names it once. The Clause by clause section lists the clauses consulted.

Most of it is built as the page is laid out. What stays with you is the outline: which part of the document is a section, which picture means what, and in what order it should be read.

Clause by clause

The engine reports a missing tree, missing table headings, a missing title and a missing language, then refuses to write: a document that claims the accessibility standard without holding to it does not come out of the engine. That report is asked of the engine before writing — through the service, the check key of the request — and it lists those gaps while the file does not yet exist.

The second part of the accessibility standard asks that a way into the document name a piece of its structure rather than a page. The engine writes those destinations: a link, a bookmark or the opening view leads to an element of the tree, and the page that element begins on is written beside it for reading software that knows nothing of the structure. A document carrying one is written as PDF 2.0, and an identifier no element of the tree carries is refused rather than written. The report also names ways in that lead to a page without naming an element of the structure.

The engine asks the calling program to mark decoration as decoration; it does not check that marking. It reads and reports the table rules: the shape of the table, one caption at one end of it, rows of the same width, no cell laid over another, and every heading cell that either says what it heads or is named by the cells that use it.

None of those rules reaches a table cut by a page break. ISO 14289-1 7.5 and ISO 14289-2 8.2.5.26 ask which headings a cell answers to and what shape a table has; neither says a word about a heading row drawn again on the following page, and no published text of the family names such a repeat. What is written here is therefore stated rather than cited: a heading cut across two pages is one heading, tied into the structure once, and both drawings of it are read as that same heading. No published rule lets a checker measure that choice; it is therefore named here.