Accessible lecture slides: contrast, alt text, structure
Accessible lecture slides come down to five habits: text at 4.5:1 contrast and at least 20 px; alt text on every figure that says what it shows; colour never the only carrier of meaning; a real heading on every slide; and a text handout alongside, because a PDF of slides is not a document a screen reader can navigate. It needs the habits and one check before the term, not a special tool.
This guide covers each habit with the check, what Marp's exports carry, and the handout that closes the gap.
Contrast and size
- Body text 4.5:1 against the background, large headings 3:1 (WCAG's thresholds). Dark grey on white passes; mid grey on white does not; most pastel accents on white do not. Check a theme's colours once with any contrast checker and stop guessing.
- Body text 20 px or larger on a 1280×720 canvas, headings 36 px or larger, nothing students must read below 16 px. The back row and a student with low vision want the same thing. See Marp font size.
- Light background for the room: dark themes lose contrast on projectors.
- Line length under about 70 characters; two-column layouts help.
Colour is not the signal
A chart whose series differ only by colour, a table whose highlighted row differs only by tint, a "red means wrong" convention: each is invisible to someone with colour-blindness (about one in twelve men) and to a monochrome printout.
- Add a second signal: a label, a marker shape, bold, an icon (✓ / ✗), a pattern.
- Use a colour-blind-safe palette for charts (avoid red/green pairs; blue/orange is safe).
- Say the signal aloud: "the bold row is ours".
Alt text on figures
Every image gets alt text that says what the figure shows, not what it is:  rather than . Marp keeps the alt attribute in the HTML export and in the slide's markup; keywords such as w:600 are stripped from it, so the sentence survives. Decorative images (the campus photo behind a title) get an empty alt:  has none, which is right.
Complex figures need the explanation in the handout too; an alt text is one sentence.
Structure
- One heading per slide, as the claim, so a screen reader and a sighted skimmer both get the point.
- Lists as lists, not lines with dashes in a paragraph.
- Tables with a header row and short cells; no layout tables.
- Reading order that matches the visual order (columns left then right).
- Links with text ("the scheduling chapter"), not bare URLs, and never "click here".
What the exports carry
| Headings | Alt text | Reading order | Tagged for screen readers | |
|---|---|---|---|---|
| Marp HTML export | yes (h1–h3 in each section) | yes | source order | partly: HTML semantics, but slides are SVG-wrapped |
| Marp PDF | rendered as glyphs | no | page by page | no (untagged) |
| PPTX (default, image slides) | no | no | no | no |
| Handout (Markdown → HTML or DOCX) | yes | yes | yes | yes |
The PDF and the image-based PPTX are pictures of slides. They are fine for sighted students and useless for a screen reader, which is why the handout matters: produce it from the same claim list as Markdown and convert with pandoc to DOCX or HTML, which are tagged, structured documents. Send the handout with the slides every week, not on request.
Captions and recordings
If the lecture is recorded, captions are the single largest accessibility gain and also help every student watching at speed. Most lecture-capture systems generate them; check the technical terms once per term and fix the ones the recogniser gets wrong.
The check before the term
- Run the theme's colours through a contrast checker.
- Open one deck at 4:3 on a laptop from three metres: can you read it?
- Print one deck in greyscale: does anything lose its meaning?
- Read the alt text of every figure in one deck aloud: does it say what the figure shows?
- Open the handout in a screen reader (VoiceOver on a Mac is one key) for two minutes.
Ten minutes, once, catches most of it. Then the habits do the rest.
In Lecture Studio
Decks are Marp Markdown, so alt text is part of the image syntax and headings are the slide structure; the shipped themes use dark text on white at 20 px; the Check slides scan catches overflow and missing images. The handout is a Markdown document in the unit, written from the same readings, which is the accessible artefact. The app runs on macOS 15 or later; the assistant needs an Oberik project key.
FAQ
How do I make lecture slides accessible?
Contrast of 4.5:1, text of 20 px or more, alt text on every figure, colour paired with a second signal, one heading per slide, and a text handout alongside the slides.
Are PDF slides accessible?
Not really: the PDF from Marp and most tools is untagged, and a screen reader reads it poorly. Provide the handout as a DOCX or HTML document generated from Markdown.
How do I write alt text for a slide figure?
One sentence saying what the figure shows, including the number or trend that matters. Decorative background images get no alt text.
What colours are safe for colour-blind students?
Blue and orange as a pair; avoid red/green. Add a label, marker or bold so colour is never the only difference.