Although the camelcase "viewBox" was correctly passed as a keyword argument to Brython''s `html.SVG` function, the lowercase "viewbox" is visible in the DOM. This causes the rectangle not to rescale as the browser window is resized manually.

Detailed instructions for completeness: To verify that the incorrect case breaks the viewBox functionality (tested with Brython 3.3.1 on Firefox 52.0.2 Ubuntu 14.04), right click in the centre of the webpage below and select "Inspect Element", find the svg "outer" element, and edit the attribute to read "viewBox" rather than "viewbox". The rectangle now correctly rescales when the browser window is resized manually.