问题
i have been working with HTML for some time, recently a question popped in my mind that when ever give tags
<input type = "text" name="c-id" />
<input type =text name=c-id />
with or without quotes there is no difference, when working with V-Studio, this IDE doesn't put quotes automatically, but while working with the Dreamweaver if i remember correctly it does put the quotes automatically...
what i want is to know is that at what point\s the absence of quotes makes a difference or creates a problem, and what is the best practice, always go quoted or quotes-less...
p.s. there is very good chance this question has been asked before but didn't show up in the search
回答1:
It makes a difference when the attribute value contains a space, linefeed, formfeed, or tab character, because the attribute value will end at any one of those characters if the value is not wrapped in quotes.
From an IDE point of view, it's little more than a matter of preference whether to add the quotes or not when the attribute value does not contain one of those whitespace characters.
回答2:
In XHTML syntax, the quotation marks are always required (but you can alternatively use single quotes, i.e. 'apostrophes'). This does not matter if the page is served with text/html content type, as it almost always is: browsers will parse it as sloppy HTML, not as real XHTML. But if the page is served with an XML content type, or if it is opened in a program that expects XHTML or other XML, then lack of quotation marks causes Draconian error processing: only an error message is shown, not the content at all.
In the example case, XHTML syntax is used otherwise: the “/” before closing “>” belongs to XHTML, not HTML. It makes little point to write XHTML in that respect but not consistently.
In HTML syntax, the formal rules depend on HTML version. HTML5 is much more permissive than HTML 4.01. For example, <a href=/foo/bar/ title=What???> is valid HTML5 but not valid HTML 4.01. This mostly matters in validation and depend on which version of HTML you wish to validate against (i.e., which one do you mostly try to comply with). In this issue, HTML5 reflects browser practices: browsers have long been more permissive than HTML 4.01.
By HTML5 rules, the quotes (though always allowed) are needed only when the attribute value contains any of the following characters: space, tab, line feed, form feed, carriage return, quotation mark ("), apostrophe ('), equals sign (=), less-than sign (<), greater-than sign (>), or grave (`), or is empty.
There are also opinions as well as coding style guides and other recommendations on the matter. Most people who think about such issues apparently favor the “safe” way of always putting quotes around attribute values. Some people think the quotes improve readability of code; others think they reduce it.
回答3:
Not putting your values into quotes may cause some problems, ie you can't provide multiple css classes for your elements. Browser will only understand the first value, and the page won't pass validation.
You should always put your values into quotes. The markup is easier to read and you won't run into some unexpected browser behaviour.
回答4:
For me, any practice that improves code/markup (re-)readability is worth making a habit, and having quotes on the attribute values visibly separates one property from another (readability again).
On the technical side, some attribute values have spaces and the whole tag might not work without the quotes on these. Those that come to mind: img's alt, multiple class values, title (for tooltips), an href with a url that has the equal sign (without quotes, the first equal in href= is doomed), etc.
And here is the almost same question in SO.
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My thanks to Alohci for enlightening me. I can't up vote yet.
来源：https://stackoverflow.com/questions/15827549/html-importance-of-double-quotes-in-html-tags