Hey there! Let's tackle your CSS selector questions one by one—these are super common points of confusion, so it's awesome you're diving into the details.

1. Difference between .id-name (Class Selector) and #id-name (ID Selector) for <div> Tags #

The core differences boil down to three key areas:

• Uniqueness: IDs are designed to be one-of-a-kind on a single page. You should never have more than one element with id="id-name". Classes, on the other hand, are made for reuse—you can apply .id-name to as many <div>s (or any elements) as you need.
• Specificity: ID selectors have way higher specificity than class selectors. That means if an element has both an ID and a class targeting it, the ID's styles will override the class's (even if the class comes later in the CSS). For example:

  #box { color: red; }
  .blue { color: blue; }
  

  If you have <div id="box" class="blue">, the text will be red because the ID selector wins.
• Use Cases: Reach for IDs when you're targeting a unique page element (like a main content container, header, or footer) or for anchor links (e.g., linking to #section-2). Classes are for reusable styles—think buttons, card layouts, or repeated components that need the same look.

2. Why Use #content in the Udacity Example, and How Does It Differ from .row? #

First, let's look at the context: the entire page's main content is wrapped in <div id="content">. Here's why the ID selector makes sense here:

• Unique Container: There's only one main content wrapper on the page, so an ID perfectly signals that this element is one-of-a-kind.
• Layout Foundation: The CSS for #content sets critical page-wide layout rules:

  #content { max-width: 948px; margin: 0 auto; }
  

  This centers the content on the screen and prevents it from stretching too wide on large displays—since this is a one-off rule for the top-level container, an ID is appropriate.

Now, how it differs from .row:

• Reusability: .row is a class, which means you could add this class to multiple divs if you needed more flex rows with the same display: flex; flex-direction: row-reverse style. Even though it's used once in the example, the class is built for repetition.
• Purpose: #content handles the overall page structure, while .row is a specific layout pattern (a reversed flex row) that could be reused elsewhere in the site.
• Specificity: #content has higher specificity than .row, so if any styles overlapped between them (unlikely in this case), the ID's styles would take priority.

3. What Happens If We Change #content to .content? #

It depends on whether you update both the CSS and HTML:

• If you only change the CSS selector: The styles (max-width: 948px; margin: 0 auto) won't apply to the content div at all. Your page content will stretch full-width instead of being centered, since the CSS is targeting a class that doesn't exist on the element.
• If you update both CSS and HTML: Change the CSS to .content { ... } and the HTML to <div class="content">—then everything will work exactly as before! The only differences are:
  1. You now can reuse the .content class on other divs if you want to apply that same centering/max-width style elsewhere.
  2. The selector's specificity drops. If you later add another class selector targeting this div, its styles could override .content (whereas #content would have been harder to override without using !important, which is best avoided).

内容的提问来源于stack exchange，提问作者Aal Ver