Wasn’t sure what I was looking at when I walked past a large display of thistle along the street. I did see a spider quickly vanish behind a bloom, but the white fuzzy mass did not look like spider web. This is actually non-blown collection of feather-like tufts of hair attached to the end of seeds. Each flower can produce 40-80 seeds, so with little wind to disperse the ripened seeds they were just hanging out on top the plant.
My best guess for this mass of roadside weed is that it is Canada thistle (cirsium arvense); and no, I do not welcome it in my yard,, though I actually have not witnessed any attempt for it to enter.
For more information on thistles check out: http://bulletin.ipm.illinois.edu/article.php?id=64