What's the difference between the two code formatting styles (code-fences vs indented blocks) regarding syntax highlighting?

In addition to the deprecation MattDMo pointed out here (and Nick in the comments), one small point:

Your two examples aren't quite equivalent. The precise equivalent of <!-- language: lang-js --> with code fences is ```lang-js (with the lang- prefix). ```js says "use the default formatter for the javascript tag" (because js is an alias for javascript), but <!-- language: lang-js --> tells the site explicitly to format the code as JavaScript, without referring to the tag. It doesn't matter for JavaScript, but it does for some other tags, like typescript.

Details:

The token following the ``` in code fences is a tag identifier unless you have a lang- prefix on it. It tells the site to use the default formatting for that tag, it doesn't refer to a specific language directly. To do that, use the lang- prefix.

It matters for some tags, like typescript, because the default formatting for typescript posts is JavaScript, not TypeScript (at least for now). So if you do this:

```typescript
codeHere();
```

you'll get JavaScript formatting. This gives you TypeScript formatting:

```lang-typescript
codeHere();
```

It can matter, particularly with function overloads. Here's what a ```typescript block looks like on SO (an image because the tag marker doesn't work on MSO):

Here's a ```lang-typescript block:

function exampleOverload(a: number): number;
function exampleOverload(a: number[]): number[];
function exampleOverload(a: number | number[]): number | number[] {
    if (Array.isArray(a)) {
        return a.map(v => v * 2);
    }
    return a * 2;
}