Update Jan 2021. If you get an email from google that looks like the following, if you don’t find something glaring, you can safely ignore it. IMHO it should be called spam. Imagine things to close together and to small to read on a device 3 inches by 5. Really.
Search for the above. Or css validator etc.
Many tools are out of date and give erroneous or ambiguous results. When it comes to the visual part of web work, I use a number of tools. I treat warnings as “ideas” or ‘hints’ on what you may want to fix.
Some are pretty good at finding errors. When it comes to warnings, they are not so accurate. This is especially true of mobile web warnings. There are hundreds, if not thousands of phones. Let’s say you are working on a pleasing, responsive web page with forms and pictures etc. Run a validator on it and it may complain of text to small to read or clickable elements are to close together. Wait, I ran the same evaluator on this page last year and it did not warn about anything. I run the evaluator on a different page, that last time showed warnings. No warnings this time.
I have clicked through errors reported by google. They offer this site to check your site, for errors. https://search.google.com/test/mobile-friendly I enter the site they flag for errors, and this tool says no errors.
I thank the developers of these tools. They can be very useful. But, I would take a lot of the warnings with a grain of salt. Some warnings like deprecated features, example: please consider using CSS instead of the table valign. That is a valid warning. The browsers are unlikely to drop support for the old markup yet, but, it is a valid warning. For more on this subject see https://www.quirksmode.org/css/quirksmode.html.
I have seen lots of warnings generated for a plain HTML page, that loads in less than 1/2 a second, on the number of things you could change to make the web page load faster.
When it comes to usability, I trust humans more than the validators. My first usability test is done by a guy who is 6′ 5″ and has big hands. I ask him if he can press all the buttons on my mobile pages. (Most of the time, he says, he has to use a stylus on his phone). if I get a better than most, I am happy. I use a vision impaired person and a couple of other people.
My favorite: https://www.htmlvalidator.com/
HTML / HTML5 Validator, CSS / CSS3 Validator, and More
What this guy says:
HTML5 (sometimes called just HTML) is in a constant state of flux. While many parts of HTML5 are considered stable and safe to use, there are parts that are not stable or that may not be well supported by browsers. When using HTML5, be sure to understand the potential problems when using parts and features that may not be stable (and thus may change in the future) or that may not be widely supported.
I have found this tool to be very accurate. I can get 145 warnings about a page, that is perfectly fine. I can explain and justify my response to each warning generated. When it comes to errors, it is really excellent at finding and suggesting fixes.
Windows only. The warnings can easily disabled if you don’t think they apply or are important. You have a lot of analysis options you can choose from, or ignore.
xx. Warning in line 254 at character 152: (Accessibility->General Accessibility) The alt text is the same as the title text (“Common Shoulder Sports Injuries”). The alt text and title text serve different functions and should not be the same. The alt text serves as alternative content when the image cannot be displayed. The title text is used to provide additional information and is often displayed as a tool-tip. This message is displayed up to 10 times.
xy. Message in line 254 at character 2: (Grouped->Mobile) It is recommended that the “height” and “width” attributes be used with the “img” element so browsers can leave the correct amount of space for them while downloading the image (which can take more time on mobile networks and devices). This will also prevent issues and annoyances due to reflow when the image becomes available or if image downloading is disabled. This message is displayed up to 5 times.
Here are just a few of the areas that tool reports on
“Section 508 Accessibility Standards”
“XHTML Basic Compatibility”
“XHTML 1.1 Compatibility”
“HTML 4.0 Compatibility”
“XHTML 1.0 Compatibility”
“CSS 2.1 Compatibility”
“CSS General Compatibility”
“Common Look and Feel 2.0 (Canada)”
“WCAG 2.1” if v19.01+ and WCAG 2.1 checking is enabled else “WCAG 2.0”
“Not HTML5 Compatible”
“General Compatibility (HTML5)” (for HTML5 specific elements, attributes, or other that might not be widely supported or might be problematic)
“Useless or Possibly Useless”
“Document Type Mismatch”
“Avoid Potential Conflict”
“General Compatibility (HTML 5.1)” (for HTML 5.1 specific elements, attributes, values, or other that might not be widely supported or might be problematic)
“Possibly Missing Attributes”
“May Not Be Intended”
“XHTML Feature in HTML”
“General Compatibility (HTML 5.2)” (for HTML 5.2 specific elements, attributes, values, or other that might not be widely supported or might be problematic)
“WAI-ARIA (General Compatibility)”
“Bad Property Value”
“Facebook / Open Graph”
“Section 508 Accessibility Standards (2017)”
“General Compatibility (HTML 5.3)” (for HTML 5.3 specific elements, attributes, values, or other that might not be widely supported or might be problematic)