Using Folio

Using folio

folio is a pure-Go HTML-to-PDF library. This skill captures the stable usage patterns that are easy to get wrong. It deliberately avoids enumerating which CSS properties are supported (that list changes release to release) — for the current feature surface, read examples/ in the repo and the package docs.

The one rule: use Document.AddHTML, not html.ConvertFull

AddHTML is the canonical entry point. It wires everything the converter produces into the document:

• normal-flow elements
• absolutely/fixed-positioned elements (position: absolute / fixed)
• page geometry and margins from @page rules
• margin boxes for the base, :first, :left, and :right page slots (e.g. @bottom-center { content: counter(page) } page numbers)
• document metadata from <title> and <meta>

import "github.com/carlos7ags/folio/document"

doc := document.NewDocument(document.PageSizeA4)
if err := doc.AddHTML(htmlString, nil); err != nil {
    return err
}
if err := doc.Save("out.pdf"); err != nil {
    return err
}

opts may be nil. AddHTMLWithContext is the deadline-aware variant. AddHTMLTemplate(tmpl, data, opts) runs an html/template first (invoices, reports).

Why this matters

html.ConvertFull(html, opts) returns a raw *ConvertResult. If you feed it into a document by hand, it is very easy to forward only result.Elements and silently drop result.Absolutes, result.PageConfig, and the margin-box maps. The symptom is a PDF that renders fine but is missing page numbers, headers/footers, or every position: fixed/absolute element — with no error. This is the single most common folio mistake.

Only reach for ConvertFull when you genuinely need the raw result (e.g. to inspect or adjust elements before they reach the document). When you do, add it back with Document.AddConvertResult — never wire result.Elements by hand:

result, err := html.ConvertFull(htmlString, nil)
if err != nil { return err }

// ... inspect or modify result here ...

doc := document.NewDocument(document.PageSizeA4)
if err := doc.AddConvertResult(result); err != nil { return err }

AddConvertResult forwards everything (elements, absolutes, @page geometry/margins, margin boxes, metadata) and sets the page size from any @page rule, so you don't pre-resolve geometry. AddHTML is exactly ConvertFull + AddConvertResult. Use SetPageSize/PageSize() if you need to read or override the size afterward.

Options worth knowing (*html.Options)

All optional; the zero value (nil) is "sensible defaults, all local assets must be inlined as data: URIs".

• BaseFS fs.FS — resolves every local path (images, fonts, linked CSS, background-image: url(...)). Use os.DirFS(dir), embed.FS, or fstest.MapFS. Without it, any local-asset reference fails.
• FallbackFontPath string — a Unicode TTF/OTF for glyphs outside WinAnsiEncoding (CJK, emoji, many accented scripts). Resolved via BaseFS when set.
• StrictAssets bool — turn asset-load failures (missing fonts, broken image paths, unreadable stylesheets) into a returned error instead of warn-and- continue. Turn it on in dev/CI to catch broken paths; leave off in prod.
• Logger *slog.Logger — receives warn events for failed assets when StrictAssets is off.
• PageWidth / PageHeight, DefaultFontSize, URLPolicy, Client, MaxElements.

Fonts

• The standard PDF fonts (Helvetica, Times, Courier) need no setup but only cover WinAnsi. Anything else needs an embedded font.
• Embed via CSS @font-face { font-family: 'X'; src: url('fonts/X.ttf') } with BaseFS pointing at the asset root, then use font-family: 'X'.
• For stray out-of-range characters in otherwise-Latin text, set Options.FallbackFontPath rather than restyling everything.
• See examples/fonts, examples/cjk, examples/indic, examples/rtl.

PDF/A (archival)

Set a config before saving; folio adds the XMP metadata, output intent, and /ID, and validates conformance at write time:

doc.SetPdfA(document.PdfAConfig{Level: document.PdfA2B})

Levels include PdfA1B, PdfA2B/PdfA2U/PdfA2A, PdfA3B (only level that permits file attachments), and the PdfA4* family. PDF/A requires all fonts embedded — the standard-14 fonts do not satisfy it, so supply real fonts via @font-face/FallbackFontPath. For ZUGFeRD/Factur-X invoices (PDF/A-3B + attachment + XMP schema), see examples/zugferd.

Verifying output

A folio PDF that "renders" can still be wrong (dropped content, square corners where rounding was intended, restarted list numbering, missing page numbers). Verify with Poppler tools rather than trusting that no error means correct:

pdfinfo out.pdf                       # page count, size
pdftotext -layout out.pdf -           # text + positions; grep for expected content
pdftotext -layout -f 2 -l 2 out.pdf - # just page 2 — catch content lost at a page break
pdftoppm -png -r 200 out.pdf page     # rasterize to inspect visually (bump -r for detail)

To distinguish a rounded corner (Bézier c operators) from a square one (re rectangle) that overpaints it, decompress the content stream and inspect the operators:

python3 -c "import sys,zlib,re; d=open('out.pdf','rb').read(); \
print('\n'.join(s.decode('latin1') for s in re.findall(rb'stream\r?\n(.*?)\r?\nendstream', d, re.S) \
for s in [zlib.decompress(s)] ))" | grep -E ' re$| c$' | sort | uniq -c

When fixing a rendering bug, render before (current main) and after (your branch) of the same minimal HTML and diff the two — structural unit tests alone miss visual regressions.

Pointers

• examples/ — runnable demos: hello, html-to-pdf, invoice, report, template, forms, links, merge, redact, optimize, plus the font/ script examples above. Each is go run ./examples/<name>.
• Page sizes: document.PageSizeLetter, PageSizeA4, PageSizeLegal, PageSizeTabloid, …; .Landscape() swaps width/height.
• Output: doc.Save(path), doc.WriteTo(w), and the *WithOptions variants for compression/linearization settings.

Source: carlos7ags/folio — distributed by TomeVault.