The design law of a slide page

Every slide in this runtime was drawn under a long set of rules — the canvas, the height table, the type scale, the spacing standards. The generator that drew the page had all of them in front of it. When you patch one element, you have the page's numbers and nothing else, and the rules are the only way to tell a repair from a dent.

This is that rule set, restated for editing. It is about the page, not the process: which numbers a change has to stay consistent with, and which fields actually reach the screen.

The canvas you are editing inside

The canvas is 1000 × 562.5. All elements respect a 50px margin, so the live area is left ∈ [50, 950], top ∈ [50, 512.5], and an element's right edge (left + width) stays ≤ 950, its bottom edge ≤ 512.5.

The page's own alignment grid, which existing elements are already on:

• Left-aligned content sits at left = 60 or left = 80.
• Centred content is left = (1000 - width) / 2 — recomputed, never guessed.
• Right-aligned content is left = 1000 - width - 60.

Changing width on centred content changes left too, and they are two separate writes. Read the neighbours' boxes first and land on the column they are already using; a lone element 8px off the shared left edge reads as a mistake even though nothing overflows.

There is no alignment operation. Every position is an explicit number you write to that element's own left / top / width / height, one path per call, computed from the neighbours you read. Aligning a row means giving each element the number the row already uses — not asking the page to tidy itself.

Text is sized by table, not by eye

A text element has 10px padding on all four sides, so its usable area is (width - 20) × (height - 20), and heights come from one table (line-height 1.5, padding included):

When you replace an element's words, re-derive its height instead of keeping the old one:

1. characters_per_line = (width - 20) / font_size. Keep the longest line at ≤ 75% of that; past 100% the text wraps and takes a row you did not budget.
2. Count the lines your new content needs — one per <p>, plus a wrap for every paragraph over characters_per_line — then add ~0.8 of a line of slack and round up.
3. Look the height up against the largest font size present in the content, not the average.

Height is a container, not a clamp: overflowing text spills past the box rather than shrinking, and the page shows the spill. The fix order is shorten the words, then step to the next table row, then widen the box — in that order, because a slide that needs a bigger box usually needs fewer words.

The type scale

Levels stay 2-4px apart and everything at one level uses one size. When you touch an element, take its size from the siblings that share its role on that page rather than from this table fresh — the page's own scale wins, and this table is how you recognise which level an element belongs to.

Size lives in the content HTML, as inline font-size on the <p> — there is no font-size field to write. So changing a size means writing the whole content string back, which means you have to know the markup you are replacing.

What the renderer really does with your HTML

The content of a text element is an HTML string, and this page's CSS resets most tag semantics. Four consequences decide how you write rich text:

• <ul> / <ol> render without markers. The stylesheet's reset strips list-style and the indent, and the rule that puts them back is scoped to the browser editor — not to classroom playback. A list sent as <ul><li>…</li></ul> comes out as bare unindented lines. Write bullets the way the deck itself does: one <p> per item with the marker in the text, <p style="font-size:18px;">• First point</p>.
• <h1>–<h6> are inert. The reset sets font-size: inherit and font-weight: inherit on them, so a heading tag is a <p> with extra characters. Size and weight come from inline font-size and <strong>, which do work.
• Stacked <p> tags have no gap between them in a text element — only line-height separates them. If two lines need air between them, raise the element's lineHeight or split them into two elements; do not expect an empty <p>, whose height still costs you a table row.
• A newline in plain text is not a line break. Plain text is wrapped in a single <p> and the newline collapses to a space. Multi-line content must arrive as real <p> tags, one per line.

Anything that looks like a tag is passed through as markup and not escaped, so prose containing < followed by a letter (if x<y> then) is read as HTML and mangled. Send that as an escaped entity.

Supported inline styles are font-size, color, text-align, line-height, font-weight and font-family; supported tags are <p>, <span>, <strong>/<b>, <em>/<i>, <u>. Nothing else is a contract.

Contrast is a pair, never a colour

Before you change any colour, name the two things that will sit on top of each other. Text is legible or not against its immediate backdrop, which is the shape's fill if it sits on a shape, the text element's own fill if it has one, and the page background otherwise.

Which field carries which colour:

A colour change is one write to the field the renderer actually reads — defaultColor or the inline color inside the content for text, text.defaultColor for a shape label, color for a latex or line element. Read the source JSON first and preserve its inline spans. opacity on a text element fades the glyphs and the fill together because it applies to the whole box — it is not a way to soften a background behind live text.

Aim for ≥ 4.5:1 on body text and ≥ 3:1 on 24px-and-up titles. Two calibration points from this deck's own palette: #333333 on white is about 12:1 and safe anywhere; the accent #5b9bd5 on white is about 3:1, which passes for a 32px title and fails for 16px body. The pattern that always works here is a pale tint fill with dark same-hue text — #1e40af on #dbeafe, #166534 on #dcfce7, #92400e on #fef3c7, each around 6-7:1. Recolouring one half of such a pair breaks it; recolour both or neither.

Text on a shape is one object

A label inside a card is not an independent element — its box is derived from the shape's:

text.width  = shape.width - 40            (20px padding each side)
text.height = a table value ≤ shape.height - 40
text.left   = shape.left + (shape.width  - text.width)  / 2
text.top    = shape.top  + (shape.height - text.height) / 2

Centre points should agree within 2px. So moving the shape is always two edits — move the shape, then re-derive the label — and a text height that steps up to the next table row may force the card to grow with it. The classic wrong repair is giving the label the shape's own left/top, which pins it to the top-left corner instead of centring it.

Rhythm, in exact numbers

Parallel things use identical values, not close ones: three cards in a row share one width, one height, one top, and one gap. The eye resolves 5px, so approximation is visible. When you delete one card of a row or duplicate one into it, respace all of them from the row's own arithmetic rather than dropping the newcomer next to its neighbour.

The spacing standards the page was built to:

• Title → subtitle 30-40px; title → body 35-50px; between paragraph blocks 20-30px; text ↔ image 25-35px vertically, 30-40px horizontally.
• Multi-column gap 40-60px. Any gap crossed by a connector arrow needs 60-80px, or the arrowhead lands inside the neighbouring box.
• Nothing closer than 50px to a canvas edge.

Diagnosing density from the inventory. The boxes tell you the truth without a render: sort the content elements by top and read the gaps. A crowded page shows gaps under ~20px and a bottom edge near 512; an empty one shows one cluster and 150px of dead space below it. Fix crowding by cutting words and merging elements, not by shrinking font sizes below the scale — a page whose body text has dropped to 12px is a page with too much content on it. Fix emptiness by growing the type one level and re-centring the block vertically, not by adding filler.

One kind of content, one kind of element

The page has ten element types and they are not interchangeable. Getting this wrong is the failure that renders as literal source code on a slide.

• Any mathematics is a latex element. LaTeX syntax inside a text element or a table cell renders as the raw string — the learner reads \frac{a}{b}. This holds for the small ones too (x^2, a/b).
• A latex element's rendered form is cached. Write the new source to that element's latex and the server re-renders the cached html the renderer actually paints. Verify the result with a preview.
• Table cells are plain text. No formulas, no markup.
• A code element takes plain lines, split on newlines, and renders as a fixed light card with its own border and title bar — it cannot be themed to a dark page. Its default text size is 14px, and its height must cover a 32px header plus every line.
• A line element's width is stroke thickness, not length. Length comes from start/end. Keep stroke at 2-4 (never above 6): the arrowhead is width × 3 across, so width: 60 draws a 180px arrowhead.
• Images keep their aspect ratio. Change width and height together; writing a new src does not touch the box, so an image of a different shape needs the box re-derived.

Derived elements follow their anchor

Underlines, dividers and highlight bars are shapes whose geometry was computed from the text they decorate. Move or resize the text and they do not follow — recompute them:

• Title underline: left = text.left + 10, width = text.width - 20, top = text.top + text.height + 8..12, height = 2..4.
• Section divider: width 700-900, height 1-2, with 25-35px of clear space above and below.
• Highlight bar: left = text.left - 15, top = text.top + 0.1 × text.height, height = 0.8 × text.height, width 3-6.

An orphaned rule floating beside moved text is one of the most visible signs of a careless patch.

Depth is array order

Elements paint in array order; later ones sit on top. Background shapes come before the text they hold. When something is hidden behind something else, the fix is to rewrite the element array in the order you want — never move a box to escape the overlap, and never delete the backdrop.

After the patch

Check only the elements you touched, and check these:

1. Inside the margins — left/top ≥ 50, right and bottom edges ≤ 950 / 512.5.
2. Height is a table value for the largest font size in the content, and the longest line is under 75% of characters_per_line.
3. Every colour you changed still forms a legible pair with what is behind it.
4. Text-on-shape pairs still centre within 2px; parallel elements still share exact values.
5. Any decorative shape anchored to something you moved has moved with it.
6. No LaTeX, and no raw markup, sitting inside text or table content.
7. The words still read like a slide — keywords and short phrases, under ~20 words or ~30 Chinese characters per line, no spoken sentences, and no text attributed to the teacher by name or role. Prose that wants to be spoken belongs in the page's narration, not on the page.

Where a preview is available, spend it on the changes this list cannot settle: a formula, a density judgement, or text you suspect overflows.

Where this sits

• slide-dsl is the field reference under this one — every element type's fields, which of them holds the words, what values are legal, and what the renderer actually does with each of them. It says what you may write; this skill says what is worth writing. Load it when you need a field name or a legal value.
• page-clone is the procedure for building a page out of an existing one. It decides which elements are content and which are skeleton; this skill decides whether the content you put in them is well made.
• pro-editing is the procedure for revising a page that already exists — which operation to reach for, how small to keep it, how to verify. This skill is what "well made" means once that operation lands.
• stage-design still governs the build of the stage as a whole.

Hard rules

• Nothing inside 50px of an edge, ever — recompute the number, do not nudge.
• Heights come from the table, recomputed after every text change.
• Colour is a pair. Never change one side of a text/backdrop pair alone; patch the renderer-owned colour field shown above.
• Real HTML for real structure — <p> per line, bullet characters instead of <ul>, inline font-size instead of heading tags.
• Formulas are latex elements, and their rendered form does not follow a string patch. Replace, do not patch.
• Parallel elements share exact numbers. Copy them from the inventory.
• Cut words before you grow boxes. A page that only fits at 12px is a page with too much on it.