font-synthesis-style controls permission to synthesize an oblique outline when a selected font lacks the requested style. It inherits through text spans and supports the CSS-wide keywords.

text { font-style: italic; font-synthesis-style: none; }

.oblique { font-style: oblique; font-synthesis-style: oblique-only; }

Value	Synthetic oblique permission
auto	Allows the backend's existing synthesis behavior. This is the initial value.
none	Disables synthetic oblique outlines.
oblique-only	Allows synthesis for an oblique request, and disables it for an italic request.

The property preserves face matching, glyph advances and positioning. A designed italic face remains italic with none. Full-backend native variable-font axes also remain enabled. Opposite policies can use the same loaded face without sharing the synthesized outline.

Donner currently synthesizes provider oblique outlines for its generic sans-serif and catalog faces. This control applies to that existing path in Basic and Full text. It does not add synthesis to other document fonts, add variable-font support to Basic, or promise synthesis wherever auto is specified. The property is a CSS longhand, not an SVG presentation attribute.

font-synthesis-weight, font-synthesis-position, and the font-synthesis shorthand remain unsupported. The separate font-synthesis-small-caps policy continues to control small-caps synthesis. See CSS Fonts 4, synthetic styles.

Prepared text snapshots retain the owned provider face request and classification with the encoded font bytes. Replay creates distinct faces for distinct requests, including when their bytes match, and does not consult the original provider. Font source domains use weak ownership identities, so reconstructing a source at the same address cannot reuse a destroyed source's prepared font faces. Capture admission counts copied encoded bytes and conservative fixed identity/payload storage allowances; a refused face stays invalid rather than selecting a different run face. Legacy shared fontFaces payload retention is outside this bound.

The Full backend retains its existing serialized-use requirement for face creation and destruction on its shared FreeType library. Replay-domain and metadata isolation do not change that requirement.