To center your buttons horizontally, you can use `<center></center>`: 
```html
<!DOCTYPE html>
<html>
<head>
</head>
<body>
<center>
<button type='button' style="box-shadow:3px 2px">1) My button with a shadow</button>
<br><br>

<button type='button' style="box-shadow:3px 2px 3px">2) My button with a blurry shadow</button>
<br><br>

<button type='button' style="box-shadow:3px 2px 3px purple">3) My button with a blurry purple shadow</button>
<br><br>

<button type='button' style="text-shadow:3px 2px 3px purple">4) My button with a blurry purple shadow on its text</button>
<br><br>
</center>
</body>
</html>
```
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