Understanding CSS Margins

In the given code snippet, margin: 10px 8px 15px 9px; the value used for the bottom margin is 15px. This piece of CSS describes the margins for an HTML element. The four values define the top, right, bottom, and left margins, in that order.

Margins are used in web design to create space around elements. They are a part of the box model concept in CSS which is fundamental to layout in CSS and includes content, padding, border, and margin.

Understanding the sequence

The sequence in which the margins are provided is significant.

/* CSS */
.selector {
  margin: 10px 8px 15px 9px;
}

The above declaration translates to:

• Top Margin: 10px
• Right Margin: 8px
• Bottom Margin: 15px
• Left Margin: 9px

And you can see that 15px corresponds to the bottom margin.

Practical Examples

Let's take a practical example:

Imagine you’re creating a blog page, and you're formatting the blog post titles. You may use the bottom margin to add some spacing beneath the titles to separate them from the text below.

/* CSS */
h2.blog-title {
  margin: 0 0 15px 0;
}

This will give a 15px space beneath each blog post title, providing a cleaner and more readable layout.

Best Practices

• Always keep your code as DRY (Don't Repeat Yourself) as possible. If you find that you're using the same margin over and over again in your CSS, consider creating a class for it.
• Remember that margins can collapse. In certain situations, the top and bottom margins between two elements can combine or 'collapse' into one larger margin. Being aware of margin collapse can help you debug layout issues.
• Be aware that an element's margin can’t be colored – it’s entirely transparent. This makes it perfect for creating space but not for other stylistic purposes such as background color or borders.

Having a deep understanding of CSS margin properties is crucial, whether you are making minor adjustments to the layout, or creating a new one. Gradually, usage of CSS margin properties will become second nature, and they will serve as a powerful tool in your web design kit.