The string "http://www.w3.org/1998/Math/MathML" is the MathML namespace identifier string. The fact that the URI it represents actually is resolved to this page is of no direct importance for namespace resolution purposes. This page is provided so that there is some relevant information to read in case someone wishes to do so. The matter of namespaces is discussed in the W3C documents on the matter, and MathML is explained in the current W3C MathML Recommendation.