A little knowledge about first-child

When I was reading the book "Sharp jQuery" today, I saw the section on filter selectors. There is a knowledge point that caught my attention.

(I don’t use the exact same code in the book as an example) Give a simple example - code:

1 <ul>2     <li>第一个li</li>3     <li>第二个li</li>4     <li>第三个li</li>5     <li>第四个li</li>6 </ul>

If you want the first li The color is blue, and the method given in the book is $("ul :first-child").css("color","blue");

(ps: I used jQuery is introduced online: )

My first reaction at this time was that the sentence $("ul:first-child") selected ul, not his son li. I looked for examples on the Internet and found that the common way to write this situation is: $("ul li:first-child").

Then the book Is the writing method above wrong? I tried it on the editor and found that it was correct. It turned out that the key problem occurred in ul# of $("ul :first-child") There is a space after ##! If the spaces are removed, ul is really selected!

Forehead. . . This detail. . . . Is it better to use ul+space or ul li in the future? This may be necessary haha. After all, $("ul :first-child") has one more li than $("ul li:first-child")! The conditions are even more stringent! Change the example

<ul> <p>lala</p> <li>第一个li</li> <li>第二个li</li> <li>第三个li</li> <li>第四个li</li></ul>

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