How do I set the focus to the first input element in an HTML form independent from the id?

Tag, you're it: Using HTML5 'autofocus' attribute

HTML5 introduced autofocus that's as direct as asking someone on a date:

<input type="text" autofocus> // You're it, no going backsies!
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This is the proverbial front-of-the-line pass for your input element, granting it prime focus right at the starting gate.

Hello jQuery, my old friend

Who needs an id when you have JavaScript's partner-in-crime, jQuery? Like a good detective, jQuery finds what you need:

$(document).ready(function(){
    $('form :input:visible:first').focus(); //I spy with my little eye the first input!
});
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This helps if the forms keep changing or duplicating like Agent Smith in The Matrix. It sets the focus on the first visible input element gracefully.

Will the real input please stand up: Advanced selectors

When dealing with forms screaming incognito without ids, use JavaScript with CSS selectors like Eminem in a rap battle:

document.querySelector('form input:not([type="hidden"])').focus(); // Not so fast, hidden ones. You can't hide from me!
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This line excludes the hide-n-seek champion hidden inputs, paving way for the first visible inputs to take the focus() spotlight!

Ensuring your forms take their vitamins: Accessibility

Proper tabindex is like a detailed "Connect the Dots". It provides direction:

<input tabindex="1"> //First dot connected; let the games begin!
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By giving a logical flow to your keyboard navigation, it ensures your forms are not only secure but accessible too.

The good, the bad, and the form: Best practices and potential pitfalls

Making your form a good samaritan: Accessibility

The keyboard navigability and accessibility of your forms are as important as JavaScript and HTML5! Don't forget to make your forms usable for everyone.

Dancing to the dynamism: Handle dynamically added content

For forms introduced through technologies like AJAX, make sure to target focus when the content has loaded:

$(document).ajaxComplete(function(){
    $('form :input:visible:first').focus(); // Latecomer? No problem! Welcome to the focus party!
});
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This ensures that the focus is set correctly even if web forms take a tardy ticket and arrive later than expected.

Danger, Will Robinson: Things to watch out for

When getting bolder with focus, watch out for:

• Twin Dilemma: Multiple forms? Use a more specific selector.
• Early Bird Doesn't Catch The Worm: Attempting to set focus before DOM is fully can lead to runtime errors.
• Old School Charm: Consider a fail-safe for older browsers that don't support the 'autofocus' wingman.