Supported HTML / CSS / JS

The Muonroi PDF engine renders a bounded, print-safe subset of HTML and CSS 2.1. It is a layout engine for documents — not a browser. The default policy profile is legacy-print-v1 (LegacyPrintPolicy): anything outside the subset is rejected loudly (PdfPolicyException) so a template never renders subtly wrong.

Write templates against this reference and they render deterministically on every OS.

JavaScript is not executed. There is no DOM scripting, no onload, no dynamic content. A <script> element is a hard policy rejection (see below). Compute all content server-side and emit static HTML.

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Supported HTML elements

Category	Elements	Notes
Document	html, head, body, style, title	One inline <style> (or UserStyleSheet) per document.
Block	div, p, h1–h6, section, article, header, footer, ul, ol, li	Unknown block-level tags default to display:block.
Inline	span, strong, b, em, i, a, br	<strong>/<b> bold and <em>/<i> italic via UA defaults.
Table	table, thead, tbody, tfoot, tr, td, th	border-collapse, colspan, fixed/auto layout, per-cell borders. <th> is bold + center by UA default.
Replaced	img	src must be a data-URI or resolvable via IResourceResolver. PNG/JPEG decoded in pure managed code.
Rule	hr	Rendered as a filled rule; honors height/color/background-color.
Links	a[href]	http, https, mailto only → emitted as PDF link annotations.

UA defaults applied: h1–h6 and th are bold; th text is centered; legacy IE body margin attributes (leftmargin, topmargin, …) are honored when CSS hasn't set a margin.

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Supported CSS

Box model & sizing

• margin, margin-top/right/bottom/left
• padding, padding-*
• border, border-*-width, border-* (per-side color/style), border-collapse: collapse
• width, height, min-width, max-width
• Units: px, pt, mm, cm, em, rem, %

Text & fonts

• font-family (bundled: Times New Roman/serif, Arial/sans-serif, Courier New/monospace; plus @font-face)
• font-size, font-weight (bold / ≥700), font-style: italic
• color
• line-height
• text-align (left / right / center)
• text-transform: uppercase
• text-decoration (underline / line-through)
• white-space (nowrap / pre-wrap / pre-line)
• word-break / overflow-wrap (break-word / break-all)
• Full Unicode incl. Vietnamese diacritics (font subsetting embeds only used glyphs).

Backgrounds

• background-color (solid fill — painted behind any box)
• background-image: url(data:...) (data-URI only)
• background-image: linear-gradient(...) / background: linear-gradient(...) — rendered as a PDF axial shading. Supports an angle (Ndeg/to <side>) and two or more color stops with optional % positions. radial-gradient, conic-gradient, and repeating-* gradients are not supported.
• background shorthand resolves to a solid color (or a linear-gradient, per above).

Layout & flow

• display: block, inline, inline-block, flow-root, list-item, table*
• float: left | right, clear: left | right | both
• position: absolute | relative (CSS 2.1 print positioning)
• vertical-align (notably top for table cells)
• transform: rotate(<angle>) — a single rotate() only (e.g. a diagonal BẢN NHÁP watermark); rotates the block and its text about the block center. translate/scale/matrix/skew and multi-function transforms are not supported.

Paged media

• @page { size: A4|A5|...; margin: ... } — page size + margins from CSS (options override CSS).
• @page margin boxes — @top-left/@top-center/@top-right and @bottom-left/@bottom-center/@bottom-right declare a pure-CSS running header/footer. content: may mix string literals with counter(page)/counter(pages). A programmatic options.Header/Footer overrides the matching band (API wins per band).
• page-break-before / page-break-after / page-break-inside: avoid
• counter(page) / counter(pages) — page numbering (resolved via a two-pass layout).
• @font-face (data-URI src) — subset & embedded.

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NOT supported — rejected by policy

These raise PdfPolicyException under legacy-print-v1 (with a suggested alternative in each violation):

Feature	Why / Alternative
display: flex / inline-flex	Use display:block / float / tables. (Soft-degrade can downgrade to block — see below.)
display: grid / inline-grid	Use display:table.
Flex/grid sub-properties (gap, justify-content, grid-template-*, …)	Dropped; only meaningful with flex/grid.
position: fixed / sticky	Use position:absolute or a running header/footer.
transform other than a single rotate() (translate/scale/matrix/skew, multi-function)	Only transform:rotate(<angle>) is supported.
radial-gradient / conic-gradient / repeating-* gradients	Use linear-gradient(...) or a solid background-color. (linear-gradient is supported.)
@keyframes / animation / transition	Static documents only.
External @import url(http...)	Inline the stylesheet.
<script> element	Forbidden. Render content server-side.
<a href> with javascript: / file: / other schemes	Only http / https / mailto.

Soft-degrade mode

Set PdfConfigs:Policy:SoftDegradeUnknownDisplay = true to turn display:flex/grid from a hard error into a warning — the element renders as display:block and rendering proceeds. Useful for migrating legacy templates incrementally. Default is strict (fail-loud). All other forbidden features remain hard errors regardless.

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Authoring checklist for developers

• ✅ Compute everything server-side; emit static HTML. No JS.
• ✅ Use tables and floats for layout, not flexbox/grid.
• ✅ Inline images as data-URIs, or wire an IResourceResolver.
• ✅ Use % widths for columns so layout adapts to page size; table-layout: fixed for predictable tables.
• ✅ Page numbers via counter(page) / counter(pages), in the body or in a running header/footer.
• ✅ Control page breaks with page-break-inside: avoid on rows/blocks that must not split.
• ✅ Solid background-color or linear-gradient(...) for shading (e.g. a colored/graded header band).
• ✅ Running header/footer via options.Header/Footer or @page margin boxes (@top-center { content: ... }).
• ✅ transform:rotate(<angle>) for a diagonal watermark (single rotate() only).
• ⛔ No display:flex / grid, position:fixed/sticky, non-rotate transform, radial/conic gradients, animations, <script>.

See the PDF Engine Guide for the API and a worked example, and vs DinkToPdf for what differs from a wkhtmltopdf-based pipeline.