Assertion:
More browsers historically support the SUP element than support the named character entity &trade; (trademark symbol.) For widest readability an alternative method of specifiying this symbol can be to use <sup>TM</sup> or some variation.

Using &trade; Trademark character entity:
filler text ™ filler text

Using <sup>TM</sup> :
filler text TM filler text
Using <small><sup>TM</sup></small> :
filler text TM filler text
Using <small><sup><b>TM</b></sup></small> :
filler text TM filler text