pptx-html-fidelity-audit

# PPTX ↔ HTML Fidelity Audit A repeatable workflow for catching the ways a `python-pptx` export silently drifts from its HTML source — and fixing them with a layout discipline that prevents the same regressions on the next pass. ## When this skill applies The user has: - A source HTML slide deck (typically a single-file deck with `<section class="slide">` blocks): ```html <section class="slide light"> <div class="chrome">2026 · Q2 review</div> <span class="kicker">Pillar 03</span> <h2 class="h-xl">Shipping <em>velocity</em> doubled</h2> <p class="lead">…</p> <div class="foot">page 5 / 14</div> </section> ``` - A PPTX file generated from that deck via python-pptx (or similar). - A suspicion (or visible evidence) that the PPTX doesn't match the HTML — text bleeding into the footer, italic words gone flat, hero slides not centered, sections cropped, tag styling lost. If the user only has *one* of those two artifacts, this skill doesn't apply yet — first generate the missing one, or ask the user to provide it. ## Why this is hard (and why a skill helps) PPTX is a fixed-canvas, absolute-positioned medium. HTML is a fluid, flow-based medium. A naive python-pptx export pins each block at hand-picked `(top, left)` coordinates, which works for the *first slide it was tested on* and silently fails for every other slide whose content has different intrinsic height. The result is the most common drift modes: 1. **Footer overflow** — content's `top + height` crosses into the footer row. 2. **Off-canvas content** — bottom of last block exceeds `7.5"` (16:9 canvas). 3. **Italic loss** — `<em>` in HTML never gets `run.font.italic = True`. 4. **Hero slides not centered** — vertical-stack slides use `MARGIN_TOP` instead of computing center. 5. **Box bounds intruding** — the text fits, but the *shape's bounding box* is oversized and visually crosses the rail. 6. **Tag/styling loss** — colored chrome rows, kicker uppercase tracking, mono-vs-serif assignments quietly fall back to defaults. Every one of these is a *layout discipline* problem, not a content problem. Once you adopt the discipline, they stop happening. --- ## Workflow The audit is five steps. Don't skip any of them — the discipline only works if the audit produces a real list of issues to drive the re-export. A fix-without-audit pass tends to leave half the issues alive. ### Step 1 — Extract ground truth from the PPTX Run `scripts/extract_pptx.py <path-to.pptx> > pptx_dump.json`. The script walks every shape on every slide and dumps text, position (`top` / `left`), size (`width` / `height`), and per-run typography (font name, size pt, bold, italic, color). This is the *actual* state of the export — don't trust the export script's intent, trust the dump. For 14-slide decks, the dump is ~30–60 KB and human-readable. ### Step 2 — Walk the HTML structure Read the source HTML and enumerate `<section class="slide">` blocks. For each, note: - The slide's theme (`light` / `dark` / `hero light` / `hero dark`). - The `chrome` row text (top metadata). - The `kicker` (small uppercase eyebrow above the headline). - The headline (h-hero / h-xl / etc.) and any sub-head. - The body copy and any structured blocks (pipeline steps, cards, pillars, observation cards). - The `foot` row (bottom metadata). - Any `<em>` or italic-styled spans — italic is the silent regression. Map each HTML slide to a PPTX slide index. For decks following the convention "slide 1 = cover, slide N = closing", the mapping is positional. ### Step 3 — Build the audit table For each slide, walk shapes from the dump and check against expected layout rules. Use this exact table format — the severity column is what drives the fix priority: ``` | Slide | Issue | Severity | |---|---|---| | 1 cover | meta-row 底端 6.95" 蓋過 footer (6.7") | 🔴 | | 5 checklist | row B 步驟描述底端 7.2" 切到 footer | 🔴 | | 8 3E | 收束段落直接坐在 footer 起點 | 🔴 | | 9 on-day | step 描述底端剛好碰 footer，無安全距 | 🟠 | | 多處 | em (Playfair italic) 未保留 | 🟡 | ``` Severity rubric: - 🔴 **critical** — content cropped, text invisible, footer overlap, off-canvas. Must fix. - 🟠 **high** — content visible but visual hierarchy broken, no breathing room, hero not centered. Should fix. - 🟡 **medium** — italic/em missing, font fallback wrong, color drift. Fix in this pass. - 🟢 **low** — minor spacing/alignment, sub-pixel offsets. Note but don't block. After the table, write a short root-cause section: 90 % of the issues usually come from 2–3 systemic causes (e.g. "no footer rail enforced", "hero stacks pinned to MARGIN_TOP instead of centered", "italic never propagated"). Naming the systemic causes makes the re-export script much smaller and more correct. ### Step 4 — Re-export with footer-rail + cursor-flow layout discipline This is the load-bearing technique. See `references/layout-discipline.md` for the full rules; the summary: **Define the rails up front, once, for the whole deck:** ```python from pptx.util import Inches CANVAS_W = Inches(13.333) # 16:9 CANVAS_H = Inches(7.5) MARGIN_X = Inches(0.6) MARGIN_TOP = Inches(0.5) CONTENT_MAX_Y = Inches(6.70) # NOTHING in content area may cross this FOOTER_TOP = Inches(6.85) # footer row pinned here, edge-to-edge ``` > **Customizing the rails.** The defaults above suit a 16:9 canvas with a slim footer. If your design system uses a wider footer or a 4:3 canvas, override these constants in your export script and pass the same values to `verify_layout.py` via `--content-max-y` / `--canvas-h` / `--canvas-w`. See `references/layout-discipline.md` §1 for the full constant table. **Use a cursor for content blocks instead of pinning each block at an absolute y:** ```python class Cursor: """Advances down the slide; refuses to cross the footer rail.""" def __init__(self, y_start, cap=CONTENT_MAX_Y): self.y = y_start self.cap = cap def take(self, h, gap=Inches(0.12)): # ~1 line of whitespace at 14pt; tighten/loosen per design system top = self.y self.y = top + h + gap if self.y > self.cap: raise OverflowError( f"cursor at {self.y} exceeds footer rail {self.cap}; " f"reduce block height or split slide" ) return top ``` For each slide, instantiate `Cursor(MARGIN_TOP)` and `take(height)` each block in reading order. The slide refuses to render if any block would cross the rail, so overflows become loud build errors instead of silent visual bugs. **Hero (vertically-centered) slides use a budget instead of a cursor:** ```python def hero_layout(blocks): """blocks = list of (height, gap_after) tuples in reading order.""" total = sum(h + g for h, g in blocks) y_start = (CANVAS_H - total) / 2 return Cursor(y_start) ``` That single change kills "hero slide content sticks to top" — the most common hero defect. **Tighten box height to fit text + minimal padding.** PowerPoint reveals shape bounds when they overlap (selection halos, Z-order conflicts), and an oversized box can visually cross the footer rail even when the text inside doesn't. Compute box height from text metrics + ~0.05" pad, not from generous wrappers. **Preserve italic / em explicitly:** ```python def add_run(p, text, font, size_pt, italic=False, bold=False, color=None): r = p.add_run() r.text = text r.font.name = font r.font.size = Pt(size_pt) r.font.italic = italic r.font.bold = bold if color: r.font.color.rgb = color return r ``` When walking HTML, detect `<em>` / `<i>` / inline style `font-style: italic` and pass `italic=True`. Use the EN serif face (Playfair Display, Source Serif, or fallback Georgia) for italic display copy — the CJK serif typically has no italic and looks broken if you try to italicize it. For deeper font issues that the layout rails can't catch — variable-font traps where PowerPoint silently swaps to Calibri / Microsoft JhengHei, missing `<a:ea>` slot causing CJK runs to fall back, fake-italic on Han characters — read `references/font-discipline.md`. The five layers there cover everything `verify_layout.py` can't see. ### Step 5 — Verify post-export After writing the new `.pptx`, run `scripts/verify_layout.py <path-to.pptx>`. The script: - Walks every shape on every slide. - Asserts `top + height ≤ CONTENT_MAX_Y` for content shapes (footer/page-number shapes are allowed below the rail). - Asserts `top + height ≤ CANVAS_H` for all shapes (no off-canvas). - Asserts `left + width ≤ CANVAS_W` and `left ≥ 0`. - Reports violations as a single block: slide index, shape name, observed bottom, rail. Zero violations is the gate for "this re-export is shippable". Don't claim the audit is fixed without running the verifier — the human eye misses 1–2 mm overflow at zoom-out, the script doesn't. --- ## Output to the user After Step 5 passes, report: 1. **Audit table** — the table from Step 3. 2. **Root causes** — 1-paragraph systemic explanation. 3. **Fix list** — terse list of what was changed and why (e.g. "hero slides switched to budget centering", "all content blocks routed through Cursor", "em runs explicitly italic"). 4. **Verification** — "0 rail violations across N slides, file size X KB". 5. **Path** — absolute path to the re-exported `.pptx`. The user is reading for two reasons: confirming the visible bugs are fixed, and trusting the systemic fix is right. Cover both. --- ## Bundled resources - `scripts/extract_pptx.py` — dump every shape on every slide as JSON. Run before the audit. **Important:** also run on the *original* export to compare, and on the *re-exported* one to confirm. - `scripts/verify_layout.py` — post-export rail checker. Returns nonzero exit code on violations so it slots into a CI pipeline if needed. - `references/layout-discipline.md` — the full footer-rail + cursor-flow rule set with code snippets for each common slide type (hero, content, pipeline, two-column, observation grid). - `references/font-discipline.md` — five-layer font audit: mapping, presence, variable-vs-static traps, the three XML language slots (`latin` / `ea` / `cs`), CJK + Latin italic interaction. - `references/audit-table-template.md` — copy-pasteable table template with severity legend. Read the references when: - The deck has slide types beyond what the SKILL.md covers (multi-column dashboards, embedded images, charts) → `layout-discipline.md`. - The audit shows 🟡 typography issues — italic missing, CJK falling back, unexpected `Calibri` / `Microsoft JhengHei` in the XML → `font-discipline.md`. - You want to drop the audit table directly into a report or markdown deliverable → `audit-table-template.md`. --- ## Anti-patterns to avoid - **Patching individual slides without naming the systemic cause.** If you fix slide 5 by lowering its block by 0.2", you'll be back fixing slide 9, 11, and 14 next. Find the rule that produced all four problems. - **Trusting the original export script's intent.** Always run the extractor against the actual file. Drift between intent and reality is the bug. - **Skipping verification because "it looked fine in PowerPoint preview".** Preview anti-aliasing hides 1–2 mm overflows. The script doesn't. - **Italicizing scripts that have no italic tradition.** CJK, Arabic, Hebrew, Devanagari, Thai, and Khmer all produce a synthesized slant when forced into `italic=True`, and the result looks mechanically deformed. Italicize *only* runs whose primary script supports italic — Latin, Cyrillic, Greek. See `references/font-discipline.md` Layer 5 for the implementation pattern. - **Using `MARGIN_TOP` for hero slides.** Hero slides need *budget centering*, not top-anchored. This is the most common hero defect and the cheapest to fix. --- ## Why geometry-based verification, not visual diff An earlier iteration of this skill leaned on visual diffing — render the .pptx through Keynote → PDF → PNG, screenshot the HTML through Chrome headless, stitch them side-by-side with `magick`. It worked, but with three sharp drawbacks: - **Platform lock-in.** Keynote AppleScript is macOS-only; `magick` and font-discovery commands vary across OSes; CI pipelines on Linux can't reproduce the chain. - **Imprecision.** A 1-2 mm overflow gets anti-aliased away in a PNG preview. The human eye misses it; the script catches it as a hard numeric violation. - **Setup cost.** Every contributor needs the full graphics toolchain installed before they can audit. Geometry checks need only `python-pptx`. Geometry-based verification gives up one thing the visual diff is good at: catching cases where shape positions are correct but the rendered glyph looks wrong (font fallback, kerning bugs, missing weight). When that case appears, fall back to a manual screenshot review — the five-layer audit in `references/font-discipline.md` covers most of the underlying causes.