XHTML is not very common these days, but if you do need to use it, then it seems you need to replace all ampersands ("&") with "&" according to this.
I've used the W3C XHTML validator to verify that this here is an example of a working script tag in XHTML:
Does that solve the issue you are having? If not, then please send me the XHTML you are having trouble with.
A trivial problem indeed. Solved. Thanks! Maybe you could add one line to the documentation with this minor issue, in case someone else as noob as me faces the same problem.
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