I’ve been writing Selenium tests for work lately and got tripped up recently on an element that had id “#1_ul.” Selenium kept throwing an error that the selector was invalid. I used FireFinder to generate a unique CSS selector for the element, and it returned “#\\31_ ul”. This puzzled me, until I came across this post on CSS selector escape sequences. It turns out that I needed to escape the leading digit in the selector. The way that CSS selectors work, to escape any numeric character, just prefix it with \3 and append a space character. This has to do with Unicode.