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Essential CSS Knowledge for Web Crawling

Understanding CSS is just as important as understanding HTML when it comes to web crawling.

CSS, which stands for Cascading Style Sheets, defines the style of web pages and is crucial for locating the information you need when crawling.

In this lesson, we will cover the basic concepts of CSS and the essential knowledge needed for web crawling.

Note: For more detailed information on CSS, check out the Introduction to Web Development + Build Your Own Website course.


What is CSS?​

CSS is a language that defines the design and layout of web pages, determining how the web pages will look.

While HTML is responsible for the 'structure' of the web page, CSS plays the role of styling that structure.

For example, the color of text, font size, spacing, background color, etc., on a web page are all set with CSS.


Essential CSS Knowledge for Web Crawling​

When web crawling, CSS goes beyond simply visual styling, providing hints for accurately extracting data.

With CSS selectors, you can easily find the desired HTML elements.

Let's cover the basic CSS concepts you must know when web crawling.


1. CSS Selector: Which HTML element to select?​

CSS selectors are patterns used to select specific HTML elements.

CSS Selector Structure
selector {
  property: value;
}

For instance, to select all <p> tags on a website, you use a selector like this:

Basic CSS Selector Example
/* Select all p tags */
p {
  /* Set the text color of all paragraphs to green */
  color: green;
}

2. Class and ID: Attributes to differentiate HTML elements​

Class and ID are attributes used to differentiate HTML elements, often used when applying CSS styles.

A class can be applied to multiple elements, whereas an id must be unique within a page.

Class and ID Example
<div class="product-item">Product 1</div>
<div class="product-item">Product 2</div>

<div id="header">Header</div>

In the code above, all div elements with class="product-item" will have the same style applied by CSS.

On the other hand, the div element with id="header" will have a unique style applied.

In CSS, you use . to select elements by class, and # to select elements by id.

Basic CSS Selector Example
/* Select elements with class="product-item" */
.product-item {
  /* Set the font size of all product items to 14px */
  font-size: 14px;
  /* Set the text color of all product items to blue */
  color: blue;
}

/* Select the element with id="header" */
#header {
  /* Set the top margin of the header to 10px */
  margin-top: 10px;
  /* Set the background color of the header to yellow */
  background-color: yellow;
}

Especially when web crawling, it is often necessary to specify particular elements using the class or id attributes.

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