R Academy · Lesson

HTML Structure and CSS Selectors

Understand DOM trees and write CSS selectors to target elements.

The HTML DOM Tree

Every web page is structured as a Document Object Model (DOM) tree. HTML elements nest inside each other forming a parent-child hierarchy. Web scraping navigates this tree to extract data.

# HTML structure conceptually:
# <html>
#   <body>
#     <div class='container'>
#       <h1 id='title'>Hello</h1>
#       <p class='text'>World</p>
#     </div>
#   </body>
# </html>

# rvest lets us query this tree with CSS selectors
library(rvest)
html <- read_html('<div><h1>Title</h1><p class="info">Text</p></div>')
html_text(html_element(html, 'h1'))

CSS Selector: Tag Selector

The simplest CSS selector targets elements by their tag name. Writing p selects all paragraph elements. Writing h1 selects all level-1 headings.

library(rvest)
html <- read_html('
  <div>
    <p>First paragraph</p>
    <p>Second paragraph</p>
    <h2>A heading</h2>
  </div>
')
# Tag selector: selects all <p> elements
nodes <- html_elements(html, 'p')
html_text2(nodes)
# Returns: c('First paragraph', 'Second paragraph')

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