CSS for the XAML Guy - Positional Selectors


AngleBracketsI got into a longish, public discussion last night about XAML versus the HTML/CSS stack last night. I think they both have merit and pros and cons but it made me decide to add a short series of posts that highlight some of the CSS things that surprised me most (like my JavaScript for the C# Guy posts - and yes, more of those are coming too).

The first topic I am covering is some subtleties of the selector syntax. CSS selectors allow you to pick children, descendants and adjacent siblings. I found that I used descendant selector quite a lot:

#main h1 /* descendant */
{
  color: Red;
}

This syntax effectively says, any h1 inside an element named "main". I see people assume this actually means, *directly* inside the "main" element. So if you had some HTML like this, all h1's inside "main" would be colored red:

<!DOCTYPE html>
<html>
<body>
  <h1>Top</h1>
  <section id="main">
    <h1>Main Level</h1>
    <section id="article">
      <h1>Second Level</h1>
      <p>Yup</p>
    </section>
  </section>
</body>
</html>

Note that it applies to all the descendants. But but if you did want just the h1's directly inside (or...ahem a child) of "main"? That's child selector syntax:

#main > h1 /* child */
{
  font-size: 24px;
}

Then only the h1 containing "Main Level" will have the rule applied. I've seem some CSS with tons of "!important" to override the descendant syntax because they didn't seem to know about child selector. 

The last one is the adjacent sibling selector. This selector looks for elements that are directly next to each other (or...ahem...adjacent):

h1 + p /* adjacent siblings */
{
  border-bottom: black 2px solid;
}

In this case, the p (not the h1) will have a border under it. The other h1's will be left unmolested. Here's a complete example if you want to play:

"If you're not careful, you just mind learn something..."


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