- *canvas* is the surface that the SVG is rendered on

- *viewport* is the region of the canvas within which SVG is rendered

- the size of the initial viewport is a "negotiation" between SVG 
  user agent and parent user agent.
  + width of root <svg> gives the viewport width UNLESS ...
    - SVG is embedded by reference (e.g., <object>) or inline AND ...
      + the referencing element or parent document has CSS styling AND ...
        - that styling is sufficient to determine the viewport width
  + ditto height

- intrinsic width and height:
  + from 'width' and 'height' attributes
    - if not specified, interpreted as "100%"
  + percentage width and height do NOT provide intrinsic width/height
    AND do not indicate percentage of containing block
    + they indicate portion of the viewport that is covered by image data

- intrinsic aspect ratio:
  + 'width'/'height' if they are both specified (and not percentages)
    - otherwise from width and height in the 'viewBox'
      + otherwise unknown (or 2:1 in SVGv2)

- 'viewBox' provides x-left, y-bottom, width, and height to map
  user space to bounds of the viewport
  + can be affected by 'preserveAspectRatio' (to enforce uniform scaling)