外部 CSS 与内联样式的性能差异？

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Well it can but the reason for the linked or external style sheet is so it can be cached in the browser especially when your using the same div in multiple pages for the site. This means the browser only has to load the style sheet once instead of having to reload the code every time the browser reloads page. It also makes for cleaner code which makes any changes or debugging easier.

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最佳答案

The performance boost that your friend mentioned is probably too trivial compared to the amount of performance boost (through other factors) using a CSS file.

Using the style attribute, the browser only paints the rule for that particular element, which in this case is the <div> element. This reduces the amount of look up time for the CSS engine to find which elements match the CSS selector (e.g. a.hover or #someContainer li).

However, putting styling at element level would mean that you cannot cache the CSS style rules separately. Usually putting styles in CSS files would allow the caching to be done, thus reducing the amount of load from the server each time you load a page.

Putting style rules at the element level will also make you lose track of what elements are styled what way. It might also backfire the performance boost of painting a particular element where you can repaint multiple elements together. Using CSS files separates the CSS from HTML, and thus allows you to make sure that your styles are correct and it's easier to modify later on.

Therefore if you look at the comparison, you would see that using a CSS file has much more benefit than styling at element level.

Not to forget when you have an external CSS stylesheet file, your browser can cache the file which increases your application efficiency!

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THE TRUTH IS 'YES'

There is a huge difference. Especially when you are automating user interface. Try the following code. I use IE10 and notepad to develop. I am learning as I go and make tests this is a shortened version test. (there maybe errors as I reduced the code to show your answer)

Click on the image you reference and read the alert messages. HINT: Save this file the save again as an edit before editing (testing).

Best wishes, Don

<!DOCTYPE html>
<head>
<style>
div.grid
{
width:180px;
height:42px;
border:none;
}
img
{
width:50px;
height:50px;
margin:2px;
float:left;
border: 1px solid red;
}
</style>
<script>
function handleSelect(xId)
{
//
// TESTPOINT
alert("TESTPOINT\r>Grid: " + xId);
//
// GET BORDER COLOR
// NOTE: An empty or blank value when you can see a border means the tag itself does not
//            have 'border properties' (style="border: 2px{width} solid{style} green{color}").
//            although there can be a border detailed via css local or external or via code (script).
//            If the 'border properties' are returned then they were setup at the tag as
//            above or the 'border properties' were updated by script code not css code.
//            If the 'border properties' are NOT returned then they were setup via css.
//            Thus, since everything seems to be heading toward edit on the fly (live) then css is NOT the way to go (learning).
// HINT: Margin property is also not readable if set via css. Most likely all the properties values are the same way.
//           Thus, setting the property values of a tag should be set at the tag control.
// (works) cBorder=document.getElementById(xId).style.borderWidth;
// (works) cBorder=document.getElementById(xId).style.borderStyle;
// (works) cBorder=document.getElementById(xId).style.borderColor;
// (works) cBorder=document.getElementById(xId).style.border;
//cBorder=document.getElementById(xId).style.border;
cBorder=document.getElementById(xId).style.margin;
alert("TESTPOINT\r>Grid: " + xId + "\r>Border: " + cBorder);
//
// SELECT IMAGE
document.getElementById(xId).style.margin="1px";
document.getElementById(xId).style.border="2px solid gold";
document.getElementById(xId).innerHTML=xId;
alert("TESTPOINT\r>Grid: " + xId + "\r>Border: " + cBorder + "\r>[set border color gold]");
//
// GET BORDER COLOR
//var cBorder=document.getElementById(xId).style.border-Color;  //Error
//var cBorder=document.getElementById(xId).style.border-color;  //Error
//var cBorder=document.getElementById(xId).style.borderColor;   //Error
//var cBorder=document.getElementById(xId).style.bordercolor;   //Undefined
cBorder=document.getElementById(xId).style.border;      //Empty
alert("TESTPOINT\r>Grid: " + xId + "\r>Border: " + cBorder + "\r>[set border color gold]" + "\r>Border: " + cBorder);
}
</script>
</head>


<body>
<div class="grid">
<img style="border: 2px solid green" id="R0C0" src="someimage.bmp" onclick="handleSelect(id)">
<img style="border: 2px solid blue" id="R0C1" src="someimage.bmp" onclick="handleSelect(id)">
<img style="border: 2px solid purple" id="R0C2" src="someimage.bmp" onclick="handleSelect(id)">
</div>
<div class="grid">
<img id="R1C0" src="someimage.bmp" onclick="handleSelect(id)">
<img id="R1C1" src="someimage.bmp" onclick="handleSelect(id)">
<img id="R1C2" src="someimage.bmp" onclick="handleSelect(id)">
</div>
<div class="grid">
<img id="R2C0" src="someimage.bmp" onclick="handleSelect(id)">
<img id="R2C1" src="someimage.bmp" onclick="handleSelect(id)">
<img id="R2C2" src="someimage.bmp" onclick="handleSelect(id)">
</div>
</body>
</html>

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It's not an easy question to answer, because the perfomance in this case depends on many factors (complexity of CSS selectors, document size, etc.). However, if we take an isolated case, then we can see that CSS class is in general faster than inline style:
Inline style vs CSS class

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Using external style sheets is definitely a better option because it will help you to remember the style you have applied on the div(s). It reduces the time of loading the page because the lesser the HTML code the faster it will load.

But in some cases you might have to change some property of a particular div then the inline style is the best option. And truly speaking, one or two inline style won't make any change the time of loading the page.

There is another option of internal style sheet but it is used only when you have a single page website like if you are making a template. This is because you have to write CSS in every HTML page

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The page will load faster if you use inline styles vs style sheets. In some cases must faster.

When you use a style sheet using href it requires another request to the server, then the parsing of the file after response. With inline styles there is none of that, just direct parsing.

If a client has slow internet then that single request could be very slow leaving the page style-less until the style sheet get delivered. Again, if it were inline there would be no delay at all.

The only reason we use style sheets is to be organised. There are times when they are not needed, so inline styles or in-document style sheets suffice.

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I prefer using inline CSS over external CSS where there are multiple small CSS files for every other element or image. No point in downloading several CSS files with merely 5-10 lines of code in each. If your element contains properties such as hover, active, checked etc. you're then supposed to use an external CSS file as it will avoid complicating your development process.

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There is no fixed answer in my opinion.

An inline CSS will load faster if the CSS content size downloads faster than your server would respond to an external CSS file request (considering DNS time, server latency, etc).

For normal size CSS I would inline them in the page, for something over 15-20KB I would probably put it in an external file and make sure it can be cached.

I am sure there are many other considerations I am missing now, but there is no fixed answer for inline vs external.

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Definitely yes, as there's no server requests the css is loaded and rendered together with the html, making it much faster.

But I recommend using internal css instead of inline css, using the <style> inside the <head> tag with your core styles needed to display the homepage and then loads external CSS files