Set this CSS Dimension to the specified number of x-heights.
One ex is equal to the x-height of a font's baseline to its mean line, generally the height of the letter "x" in the font, which is usually about half the font-size.
Dimension.ex(this._value) : _unit = 'ex';
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https://api.dartlang.org/stable/2.0.0/dart-html/Dimension/Dimension.ex.html