The WebCore/css/mathml.css stylesheet contains the following comment: /* FIXME: For a RenderMathMLOperator's margin-start and margin-end, or for a MathML "embellished operator", check the operator's lspace and rspace attributes, and the MathML operator dictionary. */ math > mo, mrow > mo, msqrt > mo, mtd > mo { -webkit-margin-start: 0.2em; -webkit-margin-end: 0.2em; } basically, we always add .2em before and after the <mo> element. We should use the default lspace/rspace values given in MathML operator dictionary instead: http://www.w3.org/TR/MathML/appendixc.html#oper-dict.entries-table which are multiple of 1/18em: http://www.w3.org/TR/MathML/appendixc.html#oper-dict.space Also, the we could use the lspace/rspace attribute valus when specified: http://www.w3.org/TR/MathML/chapter3.html#presm.mo.attrs In theory, the spacing should be added to around the whole embellished operators not necessarily to the core mo, but I'm not sure how much Webkit implements about the embellished operator hierarchy.