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Travelogue · Namespace Edition

The Foreign Quarter

A multimedia tour through everything the HTML parser does not natively own

Six districts. Five namespaces. One overworked compiler.

Every neighbourhood on this page lives just outside the comfortable streets of plain HTML. Some are governed by the SVG council. Others by MathML. A handful by custom-element guilds. A few are simply lawless. We document what survives the trip home through Atlas.

District I

The SVG Arcade

A glassy quarter of filters and gradients. Animations loop overhead in SMIL. A door of <foreignObject> lets HTML wander inside an SVG and out again.

An exhaustive SVG: filters, masks, clipPaths, gradients, patterns, symbols, animations, text, and one cursed door.

District II

The MathML Scriptorium

Equations as first-class document children. The browser renders them; the Atlas compiler may not.

Euler's identity, in native MathML:
The quadratic formula:
A 2×2 matrix, because MathML still ships <mtable>:

District III

The Custom-Element Guilds

Hyphenated tags wearing unfamiliar attributes. Some carry a shadow root in their pocket.

Press for adventure

A Hero Section by Another Name

Custom elements behave like HTMLElement in jsdom by default. We expect them to traverse — but their unknown attributes are likely dropped.

Learn more
Ada Lovelace

First programmer. Translator of Babbage. Patron saint of foreign quarters.

Slot fallback content — this slot is unattached and should render its fallback.

District IV

The Picture Gallery

Six sources, two formats, four breakpoints. Every <source> in this matrix is a stress test for art-direction preservation.

A panoramic view of the foreign quarter at dusk, lit by mixed gradients.
Six sources of art-directed responsive imagery. Type and media attributes must round-trip exactly.

District V

The Media Houses

Video, audio, embedded objects, iframes — each its own protocol for losing or surviving the trip.

Your browser cannot play this video. Download it instead.

Three sources, three subtitle tracks, an HTML fallback paragraph.

No audio support detected. Sorry.

An audio walking tour, encoded three ways.

Your browser would not load the SVG diagram. Here is the prose version: a hexagonal lattice of districts, connected by lines of varying thickness.

An <object> with structured fallback prose.
An <embed>, present for archaeological purposes.
An iframe whose entire document lives inside its srcdoc attribute.
A sandboxed iframe with selective allow-list.
An experimental <portal> — a tag the HTML parser may or may not have heard of.

District VI

The Newer Neighbourhoods

Tags so recent the parser may treat them as unknown elements. Some are real now; some are still on the drawing board.

A Welcome Dialog

The native <dialog> element with the open attribute should render inline.

The SVG Arcade

The MathML Scriptorium

The Custom-Element Guilds

The Picture Gallery

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District VII

The Lawless Strip

Markup that should not appear in the body of an HTML document at all. We include it on purpose.

A namespaced element with namespaced attributes.

The surrounding <div> declares two XML namespaces. The child uses one as a prefix.

End of the tour. Whatever survived the journey home is what Atlas considers worth keeping.