How to avoid the problem of 'flash of unstyled…

Most effective methods:

Synchronous CSS loading: Place <link rel="stylesheet"> in the <head> of the document so that the browser loads and applies styles before rendering content.

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>My App</title>
  <link rel="stylesheet" href="/styles/main.css">
  <!-- Other head tags -->
</head>

Inlining critical styles: Inline styles necessary for displaying the visible part of the page ("Above the fold") directly in <head> using the <style> tag. This speeds up initial rendering. Other styles can be loaded asynchronously.

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>My App</title>
  <style>
    /* Critical styles here */
    body { margin: 0; padding: 0; }
    h1 { color: blue; }
  </style>
  <link rel="stylesheet" href="/styles/main.css">
  <!-- Other head tags -->
</head>

Using font-display: swap or similar properties for fonts: For custom fonts that may cause FOUT (Flash of Unstyled Text), use font-display in the @font-face CSS rule. swap allows the text to be displayed immediately with a system font and replaced with the custom font after loading.
```
@font-face {
  font-family: 'MyCustomFont';
  src: url('my-custom-font.woff2') format('woff2');
  font-display: swap; /* Or fallback, optional, block */
}
```
Applying lightweight templates (in server-side rendering): If server-side rendering (SSR) is used, embed a minimal set of styles necessary for displaying the page skeleton into the HTML.
Minimizing and combining CSS files: Reducing the number of HTTP requests and the size of CSS files speeds up their loading.
Using build tools: Webpack, Parcel, and other bundlers can automate critical style inlining and CSS optimization.

How to avoid the problem of 'flash of unstyled content' (FOUC)?