The internet is full of tools that can scan your website, or a specific page on your website and let you know what improvements you can make. Some scanners find Search Engine Optimization (SEO) issues. Some find security issues. Some find speed/optimization issues. Some find accessibility issues that will help you improve your website for people that rely on screen readers or suffer from color blindness. Many do some combination of all of those.
These scanners can be a great help to you, and web developers like us, but as with many things you also have to take some things they're telling you with a grain of salt.
A perfect score on any given test is probably not going to happen, and in many cases would actually be a *bad* idea to fix. In fact, some of the most popular websites made as the result of millions of dollars of development get dismal scores on these tests.
So the question is, how do you know what to fix, and how do you know when to leave well enough alone? The short answer is that you won't always, and some of the issues are entirely out of your hands.
If a suggestion makes you say "huh?" skip it and fix what you can. With many of these scanners fixing a few obvious issues will get you a lot farther than small technical tweaks.
The Common Issues
A lot of the bigger issues hurting you with these scanners will be your images.
Much like how when you open a picture on your computer or phone you can still zoom in to see its actual size, a picture on a web page can be shown smaller than it really is.
This means the computer downloads a large image, and then shows it small, which is wasted time and bandwidth. In the case of people on mobile connections with limited data it's even a waste of money.
If you're showing a picture as 300x200 on the page, make sure the image is actually 300x200 in a photo editor.
While you're in that photo editor make sure you use a "save for web" option if possible.
This will help fix the other issue almost all scanners will ding you for, unoptimized images. It's a bit technical but basically digital photographs of the same height and width can be stored with different levels of detail. You can take the an image, drop some detail, and get a much smaller file size while hopefully maintaining a good enough image. Often times the image is almost imperceptibly different to the eye, despite being 50-70% of the original size.
Almost all scanners will ding you if your css and js files are not minified. That more or less means that any parts of the file that are only there to make them human readable (usually whitespace) are removed. That way the file every device downloads is less data, much like image optimization.
Although there are many scanners, here are a couple we most frequently use.
This tool is provided by Google and will give your site a score from 0 to 100 for both mobile devices and desktops. Red is bad, green is good, and yellow is in between. The scores for both desktop and mobile can vary quite a bit. For example as of this writing Amazon.com gets a 42 (red) for mobile, and a perfect score of 100 for desktop.
This test does tend to focus on the technical aspects of your page, but not everything about it has to be solved by a technical person. For each suggestion there is a link to figure out what you would do to correct it.
This scanner used to have the excellent feature of creating the optimized versions to images for you but, for now at least, that feature is gone.
This scanner is free.
This scanner is an extension for Chrome. It will scan multiple pages on your site and look for any number of issues. Where Pagespeed Insights will tell you all the technical aspects to make your website better technically, Checkbot is very good at finding areas where you can improve your website SEO wise, and while doing, for users. It will make sure all your pages have distinct/unique titles and descriptions set. Make sure none of your pages has duplicate content. It will also alert you to any “thin content” pages, meaning pages that don't have much on them content wise. While all these things are good to fix for SEO reasons, they'll also just make your site better for everyone.
This scanner has a free option and a paid option.