When working with scoped CSS in Vue, applying styles from a parent to a child component can be tricky. Vue offers two powerful tools to help with this: the deep selector (>>>) and the :deep() pseudo-class. Both allow you to penetrate scoped CSS boundaries, but which one should you use? Let’s explore both with examples and compare them!

🛠 1. Using the Deep Selector (>>>)

As we discussed earlier, the deep selector is a special way to apply styles from a parent component to a child component while keeping the scoped attribute in place.

Example:

<!-- ParentComponent.vue -->
<template>
  <div class="parent-container">
    <ChildComponent />
  </div>
</template>

<script>
import ChildComponent from './ChildComponent.vue'

export default {
  components: {
    ChildComponent
  }
}
</script>

<style scoped>
/* Scoped styles apply only to the parent */
.parent-container {
  background-color: lightblue;
}

/* Deep selector to apply styles to a child component */
>>> .child-text {
  color: red;
}
</style>

In this example, the deep selector (>>>) allows us to style the .child-text class inside the child component, even though both components have scoped styles.

🛠 2. Using the :deep() Pseudo-Class

Vue 3 introduced a new and more semantic way of applying deep styles using the :deep() pseudo-class. This is the recommended way moving forward.

Example:

<!-- ParentComponent.vue -->
<template>
  <div class="parent-container">
    <ChildComponent />
  </div>
</template>

<script>
import ChildComponent from './ChildComponent.vue'

export default {
  components: {
    ChildComponent
  }
}
</script>

<style scoped>
/* Scoped styles apply only to the parent */
.parent-container {
  background-color: lightblue;
}

/* :deep() to apply styles to a child component */
:deep(.child-text) {
  color: red;
}
</style>

With :deep(), you get the same result as using >>>, but the syntax is more future-proof and aligns with modern CSS practices.

🔍 Which Is Better: >>> or :deep()?

• Vue 3 Compatibility:
  
  In Vue 3, the :deep() pseudo-class is the preferred method and will likely become the standard. It’s more semantic and readable than the >>> operator, which may feel a bit hacky.

• Vue 2 Compatibility:
  
  If you’re using Vue 2, you’ll need to stick with >>>, as :deep() is not supported in Vue 2 by default. However, with Vue-loader updates, :deep() can also be used in Vue 2 if configured properly.

💡 Recommendation

• If you're working in Vue 3, it's best to use :deep() as it’s more readable, modern, and future-compatible.
• If you're using Vue 2, the deep selector (>>>) is a reliable option. However, if your project will upgrade to Vue 3, start using :deep() to future-proof your styles.

🔧 Bonus: Combining :deep() with Inline Styles

You can also use :deep() for inline styles if necessary:

<style scoped>
/* :deep() with dynamic style binding */
:deep(.child-text) {
  font-size: 20px;
  color: v-bind('textColor'); /* Example of using Vue dynamic styling */
}
</style>

This ensures the child component gets the style, even when you bind data to the CSS dynamically!

🎯 Conclusion

Both >>> and :deep() selectors allow you to control the styles of child components while maintaining scoped styles. If you're using Vue 3, it's better to adopt the :deep() pseudo-class for a more modern, clean, and future-proof approach. However, >>> remains effective and reliable, especially in older Vue 2 projects.

Happy styling! 🎨