Use Selenium’s set_window_size(width, height) to resize the browser, then save_screenshot(path) to save a PNG for each size. Put the dimensions in a list, repeat the resize-and-capture steps, and give every file a width-specific name so one screenshot does not overwrite another. If you need an exact CSS viewport width, measure the page’s actual viewport after resizing: Selenium sets the browser window dimensions, which are not a guarantee of the same dimensions for the page’s content area.
Capture a screenshot at each size with Python
The Selenium Python API provides set_window_size(width, height) to set the current browser window dimensions in pixels and save_screenshot(filename) to save the current window as a PNG. The example below creates an output directory, captures the same URL at three illustrative sizes, checks whether each file was saved successfully, and always closes the browser.
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from pathlib import Path
from selenium import webdriver
url = "https://example.com"
viewports = [(375, 812), (768, 1024), (1280, 900)]
out = Path("screenshots")
out.mkdir(parents=True, exist_ok=True)
driver = webdriver.Chrome()
try:
for width, height in viewports:
driver.set_window_size(width, height)
driver.get(url)
ok = driver.save_screenshot(str(out / f"page-{width}x{height}.png"))
if not ok:
raise OSError(f"Could not save screenshot at {width}x{height}")
finally:
driver.quit()Install Selenium in the Python environment with python -m pip install selenium. This example uses Selenium’s Chrome WebDriver. The dimensions are examples, not a standard device list; replace them with the widths and heights your project needs. The Python API documentation consulted for this guidance is Selenium 4.50.0 documentation.
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Choose dimensions that answer your test question
For desktop responsive-layout checks, choose widths that cover the breakpoints you care about, and set a height that leaves enough page content visible for the comparison. A viewport list can contain any number of width-and-height pairs. The output filename includes both values, which helps distinguish captures even when two runs use the same width at different heights.
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Reload or resize the page already open?
The example navigates to the URL after each resize, making each capture a fresh page load at that browser window size. If your test is specifically about how an already-loaded page responds to resizing, navigate once before the loop and resize for each capture instead. The right choice depends on whether the test concerns initial loading at each size or layout changes after resizing.
Wait for the page state you need
A screenshot can be saved before asynchronous content has reached the state you intend to test. There is no universal page-ready wait that suits every site: choose a condition tied to the page, such as the appearance of a particular element, before calling save_screenshot. For a simple page, navigation followed by capture may be enough; for a page that loads content asynchronously, add an explicit wait for its relevant state. Selenium’s screenshot API documents the capture operation, but does not prescribe one universal wait for every website.
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Check the actual CSS viewport when precision matters
set_window_size controls browser window dimensions in pixels. Do not assume that the requested width equals the rendered page’s CSS viewport width: browser chrome, operating-system behavior, and browser or driver details can affect the content area. After resizing, inspect the page’s effective viewport with JavaScript:
size = driver.execute_script(
"return { width: window.innerWidth, height: window.innerHeight };"
)
print(size)If the measured content width is not the target, adjust the requested window dimensions and measure again in the environment where the screenshots will run. Keep the requested window size and measured CSS viewport distinct in logs or filenames if exact-width validation is part of your workflow.
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Use mobile emulation for device-style testing
Resizing a desktop browser window is suitable for ordinary responsive desktop checks. If the test instead requires a Chromium device-style profile, Selenium’s Chromium options support mobile emulation settings that can include screen width, screen height, and pixel ratio. These configure more than a desktop window resize, so select the approach that matches the test: responsive layout at a desktop content width, or rendering under an emulated device profile. Selenium’s JavaScript Options reference includes a custom device-metrics example.
Troubleshoot common capture problems
- Files are missing or overwritten: Confirm the output directory exists and make filenames unique for each size. The example creates the directory and includes width and height in each name.
- The save operation reports failure: Check that the process can write to the output path and that the directory is writable. Treat a
Falsereturn fromsave_screenshotas a failed capture rather than silently continuing. - The screenshot has the wrong content width: Compare the requested window size with
window.innerWidthafter resizing. The window dimensions are not a promise of an exact CSS content viewport. - Content is absent or incomplete: Wait for the page-specific element or state that should appear before capture; a successful navigation alone may not mean asynchronous content is ready.
- The result does not look like a phone rendering: A narrow desktop window and Chromium mobile emulation are different configurations. Use mobile emulation when the test needs a device-style profile, including a pixel ratio.
Or skip the browser setup
For a direct screenshot request without managing a Selenium browser session, ScreenshotNeo accepts a URL and returns an image or PDF. See the ScreenshotNeo API documentation for request options.
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curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://example.com -o shot.webpScreenshotNeo removes cookie and consent banners, newsletter popups, and chat widgets before capture; bot checks, blank pages, failed loads, timeouts, and cache hits are not billed. Its MCP server lets AI agents take screenshots, and the free plan includes 1,000 screenshots a month with no card required; paid plans start at $5 for 3,000. Sign up for free ScreenshotNeo access.
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Can I use a different browser with Selenium for these captures?
Yes. The example uses Chrome, but the resize-and-save loop is based on WebDriver methods. Use the driver for the browser configured in your test environment.
Does `save_screenshot` save a PDF?
No. Selenium’s `save_screenshot` saves the current window as a PNG. The ScreenshotNeo API can return a PDF when that output is needed.
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