There are attributes in HTML that only specify a boolean value. These include multiple, disabled, selected etc.
In XHTML, due to the strict XML syntax, you must give the attributes a value. This is normally the name of the attribute.
<select multiple="multiple">
But HTML also supports just the name of the element.
<select multiple>
And, as seen here, browsers (at least Firefox) also allow for other values with the same result.
<select multiple="yes">
Which one of these is the generally used one, or is there one? What is the official recommendation?
From the spec
A number of attributes are boolean attributes. The presence of a
boolean attribute on an element represents the true value, and the
absence of the attribute represents the false value.
If the attribute is present, its value must either be the empty string
or a value that is an ASCII case-insensitive match for the attribute's
canonical name, with no leading or trailing whitespace.
So multiple, multiple=multiple, multiple='multiple' or multiple="multiple". Nothing else (case insensitivity aside), even if browsers recover from the error.
I'd lean towards either the short (multiple) or the XML parser-friendly with the more conventional quotes (multiple="multiple").
Related
I have the following button:
<button id="=" style=width:150px;height:150px;font-size:100px; onclick="equalFunction()" disabled="true">=</button>
however in the disabled="true" my IDE recognizes the "true" value to be the "wrong attribut value" does anyone know how to solve this? The code still works, its just annoying to see it be highlighted.
You don't need to set it to true. Do something like this:
<button id="=" style=width:150px;height:150px;font-size:100px; onclick="equalFunction()" disabled>=</button>
According to the HTML5 spec:
A number of attributes in HTML5 are boolean attributes. The presence
of a boolean attribute on an element represents the true value, and
the absence of the attribute represents the false value.
If the attribute is present, its value must either be the empty string
or a value that is a case-insensitive match for the attribute's
canonical name, with no leading or trailing whitespace.
So with a boolean attribute like disabled, it's presence alone is enough and it needs no value, although virtually every web browser will still accept it. Therefore, you should use either disabled (alone) or disabled="disabled"
I have a question to the sense of the html specification at:
https://www.w3.org/TR/html5/infrastructure.html#boolean-attributes
This is clear:
The presence of a boolean attribute on an element represents the true value, and the absence of the attribute represents the false value.
But why I am not allowed to use "true" or "false" as values?
The values "true" and "false" are not allowed on boolean attributes. To represent a false value, the attribute has to be omitted altogether.
Since the presence of the attribute is enough to have a "true" value I could use ANY value like "blablabla". But why not "true" and "false"? Just to avoid confusion?
Since the presence of the attribute is enough to have a "true" value I could use ANY value like "blablabla".
No. That's invalid. You can specify either the attribute name alone, or the name and the defined value (which is the same as the name).
But why not "true" and "false"?
It's a holdover from SGML.
When boolean attributes were originally implemented they used a feature which allowed you to specify only the value.
This meant less typing and smaller, more readable files.
Obviously <input type="checkbox" checked> is much easier to understand than <input type="checkbox" true>, so the value and the name is the same.
HTML 5 doesn't define itself in SGML terms, so it says that you can specify the attribute name alone rather than the value (even though they are the same) but it needs to be backwards compatible with existing HTML parsers.
I know that a boolean attribute can optionally have a value. From the HTML5 spec:
The presence of a boolean attribute on an element represents the true value, and the absence of the attribute represents the false value. If the attribute is present, its value must either be the empty string or a value that is an ASCII case-insensitive match for the attribute's canonical name, with no leading or trailing whitespace. Note: The values "true" and "false" are not allowed on boolean attributes. To represent a false value, the attribute has to be omitted altogether.
I suppose there is a valid historical reason that explains why a value is allowed (and only allow a case-insensitive name match)... but my question is: What are the benefits, if any, of having a value for these boolean attributes? (Or disadvantages, if any.)
For example:
<option selected="selected">...</option>
...
<input required="required" ... />
versus
<option selected>...</option>
...
<input required ... />
If you need to do XHTML for whatever reason, you'll have to do that since attributes must have values in XML.
Using an empty string is consistent with having the attribute without a value. If you set a boolean attribute to empty string then the attribute will show up as unset (without a value) in Chrome dev tools.
I wonder if HTML 4 allows attributes without value, as being equivalent to attributes with an empty value. For example:
<h2 section>foobar</h2>
instead of:<h2 section="">foobar</h2>
Are the two snippets equally valid? If not, are they valid in HTML version 5?
thanks!
Boolean Attributes, Yes they are completely valid.
From W3C: (On SGML & HTML)
Some attributes play the role of boolean variables (e.g., the selected
attribute for the OPTION element). Their appearance in the start tag
of an element implies that the value of the attribute is "true". Their
absence implies a value of "false".
Boolean attributes may legally take a single value: the name of the
attribute itself (e.g., selected="selected").
This states that Boolean attributes are valid in HTML4 as well, but if you use something like, would be invalid.. because that boolean belongs to option tag.. Thanks to #Ronni Skansing for clarifying the doubt..
<p selected>Hello</p>
HTML5 Docs :
From W3C :
Empty Attribute Syntax
Certain attributes may be specified by providing just the attribute
name, with no value.
From W3C: (HTML 5.1 Nightly )
A number of attributes are boolean attributes. The presence of a
boolean attribute on an element represents the true value, and the
absence of the attribute represents the false value.
BUT
section is an invalid attribute, if you want to define your own attributes, HTML5 provides a way to do that.. you need to use data- prefix, for example, your section should be written as data-section, this way your attribute will be counted as valid.
If you hesitate to do so, we always have a validator to check - W3C Markup Validation Service
^ Validated As HTML5
NOTE: Though I provided data- is applicable for HTML5, using custom
attributes in HTML4 is invalid, no matter even if you define data-
before the attribute name, but, boolean attributes are valid in HTML4
as well.
As formally defined, HTML 4 does not allow attributes without a value. What is commonly regarded as attribute without value, as in <input checked>, is formally an attribute value without an attribute name (and an equals sign). Though misleadingly characterized as “boolean attributes” with special minimization rules in HTML 4 specs, those specs normatively cite the SGML standard.
By the SGML standard, whenever an attribute is declared by enumerating keywords that are the only allowed values, an attribute specification may, under certain conditions, be minimized to the value. This means that in HTML 4, the tag <input checkbox> is valid; the attribute is a minimized form of type=checkbox. No browser supports that (they parse checkbox as attribute name), but in validators, the construct passes.
In practice, the part of the attribute minimization rules that browsers support consists of just the special cases where an attribute is declared as allowing a single keyword value only, such as the checked attribute, which is formally declared with
<!ATTLIST INPUT checked (checked) #IMPLIED>
So it depends on how the attribute is declared in the HTML 4 spec.
But this means that the minimized attribute checked means checked=checked. The value is not empty but the keyword checked. On the other hand, browsers treat such attributes as “presence attributes”: what matters is whether an element has that attribute or not, not its value.
In HTML5 serialized as XHTML (i.e., as XML), things are simple: every attribute specification must be of the form name="value" or name='value', so the equals sign is required, and so are the quotation marks; logically, the value is always there, though it can be the empty string, as in alt="".
In HTML5 serialized as HTML, some attributes are defined so that an attribute value (and an equals sign) is not required. Rather confusingly, they are the attributes declared as being “boolean attributes” (it’s confusing e.g. because the values true and false are not allowed, but the name partly reflects the principle that the corresponding DOM property, or “IDL attribute” as they call it, has the truth values true and false as the only permitted values). For such attributes, by definition, the value is even immaterial; only the presence of the attribute matters. For example, for the checked attribute, no value is used, but if a value is given, it must be either the empty string (checked="") or identical with the attribute name, case insensitively (e.g., checked=Checked). Any other value is nonconforming but is required to work, with the same meaning (e.g., checked=false means the same as checked).
Regarding the specific example, it is not valid in any version of HTML, since there is no attribute section declared.
Both snippets are syntactically valid in html4 and html5. The first is not valid xhtml, because in xhtml an attribute value is required.
On the other hand, section is not a defined attibute, but it is a valid tag in html5. Therefore your code is not valid.
I have data attribute in html element as <button data-verified=false>Update</button>. It have boolean value for data attribute.
Is there any difference with following element <button data-verified="false">Update</button> as the data-attribute is wrapped with double quotes.
Is boolean values are supported in html?
Boolean attributes are supported in HTML, but data-verified isn't one of them, no matter how it appears in the markup. data-verified=false and data-verified="false" both create an attribute of the type string and value "false", which if tested in JS as a boolean will be treated as true
This is only the case because false doesn't contain spaces. As a contrary example, data-verified=not true is invalid and not at all the same as data-verified="not true"
There are no differences in the values - however, always prefer to quote around attribute values, because:
Looks cleaner
Easier to maintain
Every editor can deal with it easily
It's a standard, nearly all HTML code examples you'll see use the value quoted
My answer corroborates from Do you quote HTML5 attributes?
I think it is just a convention that attributes always have double quotes.
However. In jQuery, you can use the .data() method. It is smart enough to recognize booleans and numeric values.
The only difference is that only the latter is allowed in XHTML. In HTML syntax, they both are allowed, and they are equivalent: the difference is lost when the HTML markup is parsed, and the DOM contains in both cases only the string false.
This follows from general principles in HTML and does not depend on the name of the attribute in any way.
“Boolean value” is a vague term. In HTML5, some attributes are called “boolean attributes”, but this is strongly misleading – especially since values true and false, far from being the only values allowed, aren’t allowed at all for such values. You need to read the specification of “boolean attributes” to see what they really are.
When you use data-* attributes, it is completely up to you what you use as values and how you process them.