buildMathML = function(tree, texExpression, options) {
                    var expression = buildExpression(tree, options);

                    // Wrap up the expression in an mrow so it is presented in the semantics
                    // tag correctly.
                    var wrapper = new mathMLTree.MathNode("mrow", expression);

                    // Build a TeX annotation of the source
                    var annotation = new mathMLTree.MathNode(
                        "annotation", [new mathMLTree.TextNode(texExpression)]);

                    annotation.setAttribute("encoding", "application/x-tex");

                    var semantics = new mathMLTree.MathNode(
                        "semantics", [wrapper, annotation]);

                    var math = new mathMLTree.MathNode("math", [semantics]);

                    // You can't style <math> nodes, so we wrap the node in a span.
                    return makeSpan(["katex-mathml"], [math]);
                }
A map of color names to CSS colors.
TODO(emily): Remove this when we have real macros