What's The Difference Between The HTML Width/Height Attribute and The CSS Width/Height Property on The Img Element

Image width/height as an attribute or in CSS?

It should be defined inline. If you are using the img tag, that image should have semantic value to the content, which is why the alt attribute is required for validation.

If the image is to be part of the layout or template, you should use a tag other than the img tag and assign the image as a CSS background to the element. In this case, the image has no semantic meaning and therefore doesn't require the alt attribute. I'm fairly certain that most screen readers would not even know that a CSS image exists.

What's the difference between the HTML width / height attribute and the CSS width / height property on the img element?

A hot debate about the subject can be found here: Width attribute for image tag versus CSS

To sum it up:

The gain from declaring a width value and an height value (which may not be the original physical dimensions of the image) or from css declarations (like width: [valueX]; height: [valueY];) is that it helps speed up the rendering of the page. The browser knows how much space to allocate a particular area of the page: it will even draw image placeholders on a first draw, a first parsing+rendering of the page. When one does not define any width and height, then the browser has to download the image and then figure out its dimensions, space to allocate and then redraw the page.

This seems to be the most beneficial effect in my opinion.

What's the difference between HTML's and CSS's width attribute?

I checked the documents, but they don't make clear what will happen if both of them are set..

That's hidden away somewhere here:

The UA may choose to honor presentational attributes in an HTML source document. If so, these attributes are translated to the corresponding CSS rules with specificity equal to 0, and are treated as if they were inserted at the start of the author style sheet. They may therefore be overridden by subsequent style sheet rules. In a transition phase, this policy will make it easier for stylistic attributes to coexist with style sheets.

Now, as I've implied in my humorous comment, I don't know for certain why they would set a variety of values for the HTML width attribute on the img elements and have a single, different value for the CSS width property for all of them. Perhaps they're accounting for when the .thumbnail class is missing, or something else. As always, Alohci is in a better position to explain why the width and height attributes are specified anyway, even if they differ from the dimensions specified in CSS for whatever reason.

What I can tell you, however, is that this basically means the CSS does indeed take precedence anyway, even if both are set.

Should image size be defined in the img tag height/width attributes or in CSS?

I'm going to go against the grain here and state that the principle of separating content from layout (which would justify the answers that suggest using CSS) does not always apply to image height and width.

Each image has an innate, original height and width that can be derived from the image data. In the framework of content vs layout, I would say that this derived height and width information is content, not layout, and should therefore be rendered as HTML as element attributes.

This is much like the alt text, which can also be said to be derived from the image. This also supports the idea that an arbitrary user agent (e.g. a speech browser) should have that information in order to relate it to the user. At the least, the aspect ratio could prove useful ("image has a width of 15 and a height of 200"). Such user agents wouldn't necessarily process any CSS.

The spec says that the width and height attributes can also be used to override the height and width conveyed in the actual image file. I am not suggesting they be used for this. To override height and width, I believe CSS (inline, embedded or external) is the best approach.

So depending on what you want to do, you would specify one and/or the other. I think ideally, the original height and width would always be specified as HTML element attributes, while styling information should optionally be conveyed in CSS.

Is it still relevant to specify width and heigth attribute on images in HTML?

An img element has width and height attributes, but they're not required under any DOCTYPE.

Width and height attributes were only 'required' or relevant to reserve the space on the page and prevent the page moving around as it loads - which is important. This can be achieved using CSS instead providing the CSS loads quickly enough - it is likely to load before the images anyway, so all should be good.

It is also possible (and valid) to specify just one attribute, width or height and the browser will calculate the omitted value in order to maintain the correct aspect ratio.

You can specify percent values in the attributes if required. You don't need to use CSS for this, if that is what you are implying.

Also, it is relevant to add - Under HTML5 the width and height can only take a pixel value, in other words a valid non-negative integer.

Whether you use the width and height attributes can depend on your design. If you have lots of differently sized images, do you want to lump all the dimensions in the CSS or include them with the img?

Should I specify height and width attributes for my IMGs in HTML?

According to Google Page Speed, you should always define the width and height in the image tag. But, to validate you can't use the style tag.

Also, you should always specify the same height and width as the actual image so the browser doesn't have to do any modifications to it like resizing.

I'd suggest doing it

<img src="..." height="20" width="50">

Edit: Someone suggested in the comments that it would be faster to just not add any attributes. According to Google (not that they are the end all of browser knowledge):

If no dimensions are specified in the containing document, or if the dimensions specified don't match those of the actual images, the browser will require a reflow and repaint once the images are downloaded. To prevent reflows, specify the width and height of all images, either in the HTML tag, or in CSS. - Read More

Given that, you could do the img dimensions in CSS, but to validate you would have to do it in a CSS file, not inline.

BTW, Google Page Speed is a series of tips focused on rendering the page faster.

Confused with Html Image Height and Width

They are the same. No difference between CSS width and HTML width. You shouldn't style a website nowadays with HTML-tags anymore. Use CSS for this.

What are the accepted units for the <img> width attribute?

The accepted units for the width and height attributes of an HTML img element are pixels. You give the number of pixels, but don't put the 'px'.

From MDN

width
The intrinsic width of the image in pixels. Must be an integer without a unit.

It is possible to get confused between these attributes and the use of CSS properties of the same names. While the attributes had more use probably when load times were longer (they allowed the correct space for the img to be saved in the page) they may be making a comeback, see https://developer.mozilla.org/en-US/docs/Web/Media/images/aspect_ratio_mapping which ends with:

— eliminating another piece of jank from web layout! There is no need for a web developer to do anything special to their code to take advantage of this, besides returning to the habit of using width and height attributes in their HTML.

What do the width and height attributes do when using srcset and sizes?

Based on experimentation, it behaves as if you had specified width and height CSS properties in pixels. However, it can be overridden by your own CSS.

img {
  width: 200px;
  /* height will be 100px because of the HTML attribute */
}

<img
     src="http://placehold.it/100x100"
     srcset="http://placehold.it/100x100 100w, http://placehold.it/500x500 500w"
     sizes="100vw"
     alt="Sample Image"
     width="100"
     height="100"
>