css Targeting specific blocks

Hi! I'm trying to target a specific block to add a border on the left side. I have done block-specific several times on other sites but this one doesn't seem to be working. What am I do wrong?

//Start: Header Border
#block-yui_3_17_2_1_1709491059499_10305 [border-left: 1px solid black]
//End: Header Border

You have invalid [ ] symbol, it should be

#block-yui_3_17_2_1_1709491059499_10305 {
	border-left: 1px solid black;
}