Does HTML to PDF download resources from the internet?

Not by default. Without a ResourceLoader only data URIs and files under the base folder load. A loader you supply decides, address by address, what is fetched.

Which font formats work with @font-face?

TrueType, OpenType, WOFF and WOFF2. Only the glyphs a document uses are embedded.

Fonts, images and resources in HTML to PDF

Since 1.1View as Markdown

Style sheets, images and fonts load through one rule: data: URIs and files under the base folder, and nothing else unless your code says so.

Short answer

Convert a file with ConvertFileToDocument, or set BaseUri to the folder that holds the assets, and relative URLs in <link>, <img>, url() and @font-face resolve there. Fonts come from @font-face, then the Fonts collection you pass, then installed fonts; the generic families map to the standard PDF fonts. To load from the web, pass a ResourceLoader that decides per address.

string folder = Path.GetFullPath(".") + Path.DirectorySeparatorChar;
var options = new HtmlConverterOptions
{
    BaseUri = new Uri(folder),                           // fonts/, photo.jpg, logo.png and photo.webp resolve here
    UseSystemFonts = false,                              // only @font-face and the standard 14 fonts
    ResourceLoader = uri =>
    {
        // With a loader, every address comes here: decide per address, null means "do not load".
        if (uri.IsFile) return Path.GetFullPath(uri.LocalPath).StartsWith(folder) ? File.ReadAllBytes(uri.LocalPath) : null;
        if (uri.Host != "assets.example.com") return null;
        return remote.TryGetValue(uri.AbsoluteUri, out string? css) ? Encoding.UTF8.GetBytes(css) : null;
    },
};

A page converted from HTML: a photograph with rounded corners, a PNG logo, two SVG shapes, a label in Inter and a line of Arabic text

Where resources come from

Setup	What loads
No `BaseUri`, no loader (`ConvertToPdf` and `ConvertToDocument` defaults)	`data:` URIs only
`ConvertFileToDocument(path)`	`data:` URIs, and files in the HTML file’s folder and below
`BaseUri` set to a folder	`data:` URIs, and files in that folder and below, after resolving any `..`
`ResourceLoader` set	`data:` URIs, and whatever the loader returns for each absolute URL

With a loader, every URL that is not a data: URI goes to it, files included, so the loader decides for local files too. Return null for anything that must not load; the document is laid out without it. Keep the default for untrusted HTML: converting it then cannot reach internal services or read files elsewhere on the machine. A single resource larger than 64 MiB is not loaded.

@import and <link rel="stylesheet"> load style sheets the same way, as UTF-8 unless a byte order mark says otherwise.

Web fonts

<style>
  @font-face { font-family: Inter; src: url(fonts/Inter-Regular.woff2) format("woff2") }
  @font-face { font-family: Inter; font-weight: 700; src: url(fonts/Inter-Bold.woff2) format("woff2") }
  @font-face { font-family: "Noto Sans Arabic"; src: url(fonts/NotoSansArabic-Regular.woff2) }
  body { font-family: Inter, "Noto Sans Arabic", sans-serif; font-size: 10pt }
</style>

src takes TrueType, OpenType, WOFF and WOFF2 files, and local() names an installed font or one from the Fonts collection. Sources are tried in order; a damaged file is skipped for the next one.
font-weight takes one weight or a range, and font-style italic or oblique. The face whose style and weight match best is used, as CSS matches them.
Only the glyphs the document draws are embedded, as a subset.

How a font is chosen

For each family in font-family, in order:

@font-face rules of the document.
The Fonts option, a PdfFontCollection you fill from files or folders.
Installed fonts, unless UseSystemFonts is false.

The generic families map to the 14 standard PDF fonts, which need no embedding: serif (also cursive, fantasy and math) to Times, sans-serif and system-ui to Helvetica, and monospace to Courier. Common families that are not found, such as Arial, Roboto, Times New Roman or Consolas, fall back to the standard font with the same role. A document with no font named at all uses Times, as browsers default to serif.

Characters the chosen font lacks come from the next family in the list that has them, so Inter, "Noto Sans Arabic" sets Latin text in Inter and Arabic in Noto Sans Arabic. With UseSystemFonts on, installed fonts with wide coverage, such as Segoe UI, Noto Sans and DejaVu Sans, back up any character still missing.

For output that is identical on every machine, such as on a build server or in a container, set UseSystemFonts = false and ship the fonts with the application, in @font-face or in Fonts. See Fonts and text for the font engine itself.

Images

<img class="photo" src="photo.jpg" alt="Landscape photograph">
<img src="logo.png" alt="Logo" width="90">
<img src="photo.webp" alt="The photograph as WebP" width="64">
<svg width="40" height="40" viewBox="0 0 40 40"><circle cx="20" cy="20" r="18" fill="#2563eb"/></svg>

Format	Notes
JPEG	Embedded as is, without recompression
PNG	Every color type and bit depth, interlaced or not, transparency kept
GIF	The first frame, with its transparent color
SVG	Drawn as PDF vectors, sharp at any zoom, without filters, masks or patterns. Works in `<img>`, in CSS backgrounds and inline as `<svg>`
BMP	1 to 32 bits per pixel, with alpha
TIFF	The first page; fax and JPEG strips, LZW, Deflate and PackBits, 1 to 16 bit samples
WebP	Lossy and lossless, with alpha, the first frame of an animation

Images work in <img>, background-image, list-style-image and content: url(). Each image is written once per document however often it appears. Intrinsic sizes follow the pixel density a raster image records, else 96 pixels per inch. object-fit and object-position place an image in its box.

Data URIs

data: URIs work everywhere a URL does, base64 or percent encoded, so a template can carry its images and fonts inside the HTML and need no files at all:

<img src="data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='40' height='40'%3E%3Crect width='40' height='40' rx='8' fill='%2314b8a6'/%3E%3C/svg%3E" alt="">

Limits

SVG filters, masks and patterns are not drawn.
A resource larger than 64 MiB is not loaded.
With UseSystemFonts off, text needs a supplied font that covers its characters: the standard 14 fonts cover Western and Central European languages only.

Questions

Does HTML to PDF download resources from the internet?: Not by default. Without a ResourceLoader only data URIs and files under the base folder load. A loader you supply decides, address by address, what is fetched.
Which font formats work with @font-face?: TrueType, OpenType, WOFF and WOFF2. Only the glyphs a document uses are embedded.