Why is it important to specify dimensions (width, height) for images?

Specifying the width and height attributes on images matters not only for appearance, but also for loading speed, layout stability, and correct page rendering. Here is what actually happens when the dimensions are specified and when they are not.

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#1. Without specified dimensions

When the browser sees:

it does not know in advance how much space the picture will need. It starts loading the file, but for now does not reserve space in the layout.

As a result:

• first the text and other elements load;
• then, when the image arrives, the page "jumps" (shifts downward);
• the user sees a visual jump (the so-called layout shift).

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#2. With specified dimensions

Now the browser immediately knows that the picture will take up 400x300 pixels, and reserves space for it in advance, even if the image has not loaded yet.

As a result:

• the page does not jump while loading;
• text, buttons, and interface elements stay in place;
• the browser can render the layout faster.

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#3. Why this matters for performance

1. Fewer layout recalculations: the browser does not recompute element positioning on every image load.
2. CLS (Cumulative Layout Shift) is reduced: this is a page stability metric in Google PageSpeed. The fewer content shifts, the better the SEO and user experience.
3. The browser can load the page in parallel: it knows where the image will be and does not wait for its dimensions.

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#4. Dimensions can be set via CSS

If the layout is responsive, it is better to use CSS:

or through HTML + CSS:

This way the browser still knows the original proportions, and CSS adapts them to the screen width.

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#5. How the browser uses this data

• While parsing HTML, it knows the original aspect ratio (width:height).
• If the image loads later, it has already reserved a block of the required size.
• Even with loading="lazy", the content does not "jump".

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#Summary:

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In simple terms: If you do not specify width and height, the page will "jump" while the pictures finish loading. If you do, the browser will reserve space in advance, and the site will look stable, fast, and professional.