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):

  <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:

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:

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:

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:

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.