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HTML

CSS

CSS Base

Vendor Prefixing

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JavaScript

Babel includes JSX processing.

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HTML

              
                <main>
  <section>rest</section>
  <aside>150px</aside>
</main>

<main>
  <aside>150px</aside>
  <section>rest</section>
  <aside>150px</aside>
</main>

<main>
  <section>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Aperiam quod cupiditate saepe tempora incidunt, id nobis iure? Neque harum illum voluptatibus a numquam, nihil asperiores, amet provident magni minima tempore!</section>
  <aside>150px</aside>
</main>
              
            
!

CSS

              
                main {
  display: flex;
  margin-bottom: 20px;
}

section {
  flex-grow: 1;
}

aside {
  flex-basis: 150px;
  flex-shrink: 0; 
  /* flex-shrink: 0; prevents the aside from shrinking, if the section attempts to grow due to its content. (see third example) */
  
  /* Instead of flex-basis: 150px; and flex-shrink: 0; you can use the shorthand: */
  /* flex: 0 0 150px; */
}
              
            
!

JS

              
                
              
            
!
999px

Console