What Characters Are Allowed in the HTML Name Attribute Inside Input Tag

What characters are allowed in the HTML Name attribute inside input tag?

The only real restriction on what characters can appear in form control names is when a form is submitted with GET

"The "get" method restricts form data set values to ASCII characters." reference

There's a good thread on it here.

What characters are allowed in an HTML attribute name?

It depends what you mean by "allowed". Each tag has a fixed list of attribute names which are valid, and in html they are case insensitive. In one important sense, only these characters in the correct sequence are "allowed".

Another way of looking at it, is what characters will browsers treat as a valid attribute name. The best advice here comes from the parser spec of HTML 5, which can be found here: https://html.spec.whatwg.org/multipage/syntax.html#attributes-2

It says that all characters except tab, line feed, form feed, space, solidus, greater than sign, quotation mark, apostrophe and equals sign will be treated as part of the attribute name. Personally, I wouldn't attempt pushing the edge cases of this though.

what characters are allowed in the value attribute of form's input

HTML 5

Except where otherwise specified, attributes on HTML elements may have any string value, including the empty string. Except where explicitly stated, there is no restriction on what text can be specified in such attributes.

HTML 5

The value content attribute gives the default value of the input element. When the value content attribute is added, set, or removed, if the control's dirty value flag is false, the user agent must set the value of the element to the value of the value content attribute, if there is one, or the empty string otherwise, and then run the current value sanitization algorithm, if one is defined.

So there are no restrictions but the value might get altered by the value sanitization algorithm.

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For instance, if I enclose this attribute in single close, I can not safely use single quotes in it.

You can. You just can't use literal single quotes. You have to use character references.

What are valid values for the id attribute in HTML?

For HTML 4, the answer is technically:

ID and NAME tokens must begin with a letter ([A-Za-z]) and may be followed by any number of letters, digits ([0-9]), hyphens ("-"), underscores ("_"), colons (":"), and periods (".").

HTML 5 is even more permissive, saying only that an id must contain at least one character and may not contain any space characters.

The id attribute is case sensitive in XHTML.

As a purely practical matter, you may want to avoid certain characters. Periods, colons and '#' have special meaning in CSS selectors, so you will have to escape those characters using a backslash in CSS or a double backslash in a selector string passed to jQuery. Think about how often you will have to escape a character in your stylesheets or code before you go crazy with periods and colons in ids.

For example, the HTML declaration <div id="first.name"></div> is valid. You can select that element in CSS as #first\.name and in jQuery like so: $('#first\\.name'). But if you forget the backslash, $('#first.name'), you will have a perfectly valid selector looking for an element with id first and also having class name. This is a bug that is easy to overlook. You might be happier in the long run choosing the id first-name (a hyphen rather than a period), instead.

You can simplify your development tasks by strictly sticking to a naming convention. For example, if you limit yourself entirely to lower-case characters and always separate words with either hyphens or underscores (but not both, pick one and never use the other), then you have an easy-to-remember pattern. You will never wonder "was it firstName or FirstName?" because you will always know that you should type first_name. Prefer camel case? Then limit yourself to that, no hyphens or underscores, and always, consistently use either upper-case or lower-case for the first character, don't mix them.

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A now very obscure problem was that at least one browser, Netscape 6, incorrectly treated id attribute values as case-sensitive. That meant that if you had typed id="firstName" in your HTML (lower-case 'f') and #FirstName { color: red } in your CSS (upper-case 'F'), that buggy browser would have failed to set the element's color to red. At the time of this edit, April 2015, I hope you aren't being asked to support Netscape 6. Consider this a historical footnote.

Is name attribute mandatory in input , textarea and button elements?

• If these tags are inside a form tag and you are subbmitting that
  form to a server, then name is required,
• If you are just using them for client-side purposes and don't want to send them to server, then it is optional.

Are there limits and/or prohibited characters for form field names?

The HTML5 spec defines the allowed values for name attributes on <form> elements.

The value must not be the empty string, and the value must be unique amongst the form elements in the forms collection that it is in, if any.

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Of course it also defines allowed values for the name attribute on form controls (<input>, <textarea>, &c.) (emphasis added).

4.10.19.1 Naming form controls

The name content attribute gives the name of the form control, as
used in form submission and in the form element's elements object.
If the attribute is specified, its value must not be the empty string.

Any non-empty value for name is allowed, but the names "_charset_" and "isindex" are special:

isindex

This value, if used as the name of a Text control that is the first control in a form that is submitted using the
application/x-www-form-urlencoded mechanism, causes the submission
to only include the value of this control, with no name.

_charset_

This value, if used as the name of a Hidden control with no value attribute, is automatically given a value during
submission consisting of the submission character encoding.

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