Most of what an organisation actually decides lives in documents, not web pages: policy manuals, annual reports, application forms, benefits statements, HR handbooks, course handouts. A public body's most consequential content is routinely a Word file exported once and then forgotten, not a page a designer polished.
When a document's structure fails, the people locked out are specific and predictable. A screen reader user gets either silence or a flat wall of text read in the wrong order. Someone who reflows the page to a single column on a phone loses information that depended on a layout nobody tagged. A reader with a cognitive disability loses the outline that let them skim to the one section they needed. None of this is about how the page looks; every one of these failures survives a design review that only checks appearance.
The case for fixing structure early is mostly arithmetic. Applying a heading style, tagging a table's header row, or writing alt text while the file is still open in Word takes seconds per fix. Rebuilding that same structure later, once the source file is gone or a flat export has turned the page into a picture of text, means reconstructing a tag tree by hand, page by page, in a PDF editor. Multiply either cost by a few hundred documents in a real backlog and the gap between them stops being theoretical.
Keep one real document you publish open alongside this module and apply each lesson to it directly. You will finish with a genuinely fixed file and a repeatable habit, not just notes.