The total sum of all of the advance values from rendering all of the characters within this element including the advance value on the glyphs horizontal or vertical the effect of properties kerning letter-spacing and word-spacing and adjustments due to attributes dx and dy on tspan elements. For non-rendering environments the user agent shall make reasonable assumptions about glyph metrics....of the advance values from rendering all of the characters within...‘tspan’ elements. For non-rendering environments, the user agent...