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Web Compatibility issues change over time. Bugs that were common in 2005 are not often seen in the present. This list represents the most common issues currently being discovered by the webcompat community and will be updated as these change in the future.

The Mozilla Bugzilla issues have a WebCompat priority flag from P1 to P3 following a triage process.


Current Popular Issues

(This needs to be adjusted @@karl 2020-10-20)

Bugzilla WebCompat Core Bug

Full Query
ID Summary Priority Status
91370 Implement setting cssText on various CSSRule implementations P3 NEW
102695 [EVENTTARG]Treat some transparent elements as "transparent to events" P3 NEW
129432 'onkeypress', 'onkeydown', onkeyup' not working on FORM,LEGEND,OPTGROUP,OPTION -- NEW
137687 Computed CSSDeclarations need to implement GetCssText P3 NEW
406488 Copy highlighted text to clipboard ignores non-inlined CSS styling when pasting as rich text (to HTML editors, MS Word, Excel, Outlook etc.) -- NEW
662496 autofocus not work when the element is added to the DOM after the document is done loading (e.g. in window.onload) -- NEW
695868 Unable to set document.body.innerHTML of IFrame P3 NEW
697151 Synchronous XMLHttpRequest (XHR) does not block readyState events for async XHR P2 ASSIGNED
997189 Absolutely positioned inline-block (or inline) element with floating nextSibling and auto "left" is positioned to the right of the float -- NEW
1034018 outline: auto is not applied in the same way as it is on blink P3 NEW
1039871 Add support to HTML media element for determining the mimetype of a loaded media file -- NEW
1043118 Do not select leading/trailing spaces on double clicking. P3 NEW
1199665 Consider making <input type=number> always fall back to parsing numbers as English formatted numbers if parsing them as numbers formatted using the user's locale fails -- ASSIGNED
1248570 Firefox can't play wav in video or audio element P3 NEW
1273010 Blocky and slow rendering on Firefox Android Nightly - filter: drop-shadow() P3 ASSIGNED
1274354 [meta] Date.parse, amirite P3 NEW
1307024 [Perf] [google docs] Firefox will get 3~9% faster when running gdoc test cases with chrome user agent -- NEW
1307678 Title of latest recommendation of is cut P3 NEW
1322694 WebGL video textures fail to render in Firefox Nightly for MP4 format with 2048x1024 pixels dimensions P3 UNCONFIRMED
1336245 Geolocation should return available low-res location when high-res location is not yet available P2 NEW
1337114 Diacritics rendered incorrectly with decomposed unicode and Proxima Nova P3 NEW
1342569 mouseenter/mouseleave fire differently on <select>s in e10s mode. P2 NEW
1353248 [e10s] select - options styling doesn't work properly if CSS selector :-moz-focusring is present P3 NEW
1360238 window.getSelection().toString() displays extra "\n" characters on multiple block elements, even if they are display:inline P3 NEW
1362499 Support unit-less values for CSS -webkit-perspective (for interop) P3 NEW
1362898 content with ::before gets painted at the wrong place -- NEW
1376369 -moz-user-select:none triggers dragging also when @draggable="false" P3 NEW
1382471 Firefox wraps word-broken text under a previous float-left, rather than starting it on the same line. P3 NEW
1385382 bullet with text-align:center is positioned further to the right in Firefox, as compared to other browsers (breaking fake radio button at P3 NEW
1385419 Parent element stretches to submit input's line-height, rather than its also-specified height. P3 NEW
1411862 Disable 3rd party cookie video does not play in Firefox Dev Edition (v57 with uBlockO). P3 NEW
1414402 Support playback of mp4v video P5 NEW
1417753 Implement -moz-appearance: searchfield-cancel-button which behave like -webkit-appearance: searchfield-cancel-button P3 NEW
1421597 Provide an option to ignore scroll events outside of the control when a |select| drop-down is open P3 NEW
1422891 [Rethink] Support mkv|matroska|video/x-matroska in Firefox P3 UNCONFIRMED
1436225 blocking script and cpu high with SetTimeout P3 NEW
1436801 Vertical border and padding influences alignment of text in spans with "vertical-align:middle" (only in Firefox) P3 NEW
1437761 position:sticky inside partially-offscreen position:fixed container has a different behavior in Chrome/Edge vs Firefox P3 NEW
1438398 After opening then closing the Account panel (top-right blue-ish icon), the page doesn't reflow correctly. P3 NEW
1443284 percentage height in auto-height table cell in fixed-height table is treated as auto P3 NEW
1447524 Double session history entries from P2 NEW
1449079 PNG background-image icon not positioned at the right place after first rendering P3 NEW
1450815 HTTP POST receives a response with a onload event in JS posting to itself and generating an infinite loop P3 NEW
1451998 the thumbnails all play automatically at the same time, and the website gets so slow it's impossible to use. P3 NEW
1452646 while clicking and navigating back, an outline is drawn P3 NEW
1453797 Carousel images have jagged edges when scrolling through P3 NEW
1454043 Carousel images have jagged edges when scrolling through P3 NEW
1455545 Noto Sans Symbols is not rendered on some Android devices P3 NEW
1459535 Performance is incredibly degraded when idle on P3 NEW
1461266 comments framerate is smoother on Chrome than Firefox P3 NEW
1463335 anchor and input superposed makes the JavaScript non active. P3 NEW
1464925 Visual glitch: graphic layers around menu displaced, linked to scroll position. P3 NEW
1467114 BBC - highlighter for navigation spans extends to body edge P3 NEW
1471232 iframe content overflow spontaneously after a couple of seconds P2 REOPENED
1473501 Improve the way Gecko selects images based on srcset values (don't just round up, when rounding up is a huge jump) P3 NEW
1475681 Page CSP can break seeking to unbuffered position (in mp3 playback) P3 REOPENED
1475723 Geckoview and Fennec open select dropdown on tap of parent label; other browsers do not. P3 NEW
1476051 Interop issues with select element styling with appearance:none. P3 NEW
1477110 Select elements seem to get double the text-indent P3 NEW
1481124 Menu takes longer to open in Firefox than in chrome P3 NEW
1481132 [Android] flashes 3 times when you reload or navigate away P3 NEW
1481581 [meta] More closely align the behavior of '-moz-appearance' with Chrome's '-webkit-appearance' P3 NEW
1481596 Make `-moz-appearance: none` on <textarea> behave more like Chrome P3 NEW
1486365 The City selection page slightly bounces after opening P3 NEW
1490678 Unable to subscribe to mailing list on while Tracking Protection Basic is enabled P3 REOPENED
1490873 Toggling SVG graph elements on and off leaves artifacts and erases parts. P3 NEW
1491743 invalid CSS property value on ::selection kills the default selection settings P3 NEW
1493434 Inconsistent table column sizing between Firefox and Chrome (with percent sizes on colgroup and col) P3 NEW
1495868 Unprefix -moz-available keyword P3 NEW
1499986 Handling javascript URL returning a non-string value P3 NEW
1501480 mask-image and SVG on input element fails in Firefox, Safari, and Edge, works in Chrome P3 NEW
1503427 Firefox 63 pushing Bootstrap 3 date input text down P3 NEW
1503899 Sharp printer UI incorrectly rendered, due to table-layout behavior difference (& with CSS enabled by webkit-min-device-pixel-ratio query) P3 NEW
1506487 - video not playing, OS needs to be restarted. P3 NEW
1513435 Incompatible column layout with 'height:0' and definite 'column-width' P3 UNCONFIRMED
1514269 [CSS Multicol] Firefox only web platform tests P2 NEW
1514362 [CSS Selectors] Firefox only web platform test failures P3 NEW
1514371 [CSSOM] Firefox only web platform test failures P2 NEW
1517666 quick zoom in and out will white out Firefox and all tabs P3 NEW
1518040 SVG feBlend should take account of flood-opacity on feFlood input nodes P3 NEW
1522706 History.pushState limit may be too low for some live sites P3 NEW
1523231 Screen problems not everything is there P3 NEW
1529471 objectBoundingBox, clip-path behave differently in Chrome and Firefox P3 NEW
1530577 (Un)Expected HTTPS upgrade of download due to <meta ... "upgrade-insecure-requests"> tag P3 NEW
1532481 Firefox does not expand certain list items so they do not vertically overlap, as other engines do. P3 NEW
1541330 Speech Synthesis play incorrect P3 NEW
1542995 Thousands of window.onload events creates high memory/CPU until it crashes P3 NEW
1543059 Loading spinner displays at end of some non-looping videos hosted on P3 NEW
1544428 Setting window.location.hash to an empty string on a site without a hash causes a popstate event P3 NEW
1545703 - Some videos do not play or play erratically P3 NEW
1548376 Flickering videos when scrolling on Buzzfeed P3 NEW
1550158 Blink and Gecko differ on where to flow absolutely-positioned element in a tight layout P4 UNCONFIRMED
1577904 Interop differences in iframe html/body offsetHeights as reported by Chrome and Firefox -- NEW
1656279 Unitless line height interop issue -- NEW

94 Total; 94 Open (100%); 0 Resolved (0%); 0 Verified (0%);


This is the Yeah! Let's party list.

WebKit Flexbox Total Armaggedon in China and Japan

This is not specific to Desktop and Mobile, but more to the specific markets of China and Japan and indeed more often on mobile.

Suggested Fix Use Web standards flexbox properties in addition to the WebKit ones if old devices compatibility is required.

Related Bugs


Many sites have code that detects touch support and assumes mouse input can be ignored. These sites behave oddly on all computers that have a touch screen AND a mouse - usually not responding as expected to mouse input.

Suggested Fix Write JavaScript that will handle both click and touch input, not either-or.

Related Bugs


(Fixed in Gecko) Basically an image with a max-width: 100% was bigger than the viewport It was an issue, created by a different behavior in Gecko and Blink. But the market share of Blink imposed the unspecc’ed behavior of Blink.

Suggested Fix The fix was often a combination of display: table with table-layout: fixed. This is now fixed in Gecko and the spec, but this is a good illustration point in terms of Web Compatbility issues.

Related Bugs

Video streaming using Apple’s proprietary HTTP Live Streaming

Firefox doesn’t support this on Desktop or Mobile.

Suggested Fix Not a trivial thing to fix. Ensure that there is a fallback mechanism for browsers that don’t support HLS (currently on Desktop, only Edge and Safari do).

Related Bugs

Meta viewport fixed size

Very similar to the max-width bug. This is a different of behavior in between blink and Gecko. Basically with a fix width, Gecko on Mobile doesn’t necessary resize the page to fit in the screen.

Suggested Fix Use width=device-width instead of fixed size.

Related Bugs

Faulty UA detection

Sites sniff the user agent string, often to figure out if you’re using a “supported browser”. This goes wrong all the time, because of an unknown UA string, or parsing bugs, or not understanding versions greater than 9, etc. Mobile browsers will frequently end up on “Desktop” sites, rather than mobile ones as a result of this kind of bug.

Suggested Fix Update your UA detection library or framework, or use feature detection rather than relying on User Agent string parsing. User Agent detection is about detecting the past. So it will obviously fail. Feature detection is about inspecting the now.

Related Bugs

Relying on global event object

WebKit (possibly copied from old iE?) has a non-standard global event object. When sites don’t pass in the event object to an event handler, we end up with exceptions like “ReferenceError: event is not defined”

Suggested Fix Use web standards and pass in an explicit event object into your event handlers. For example: a.addEventListener(‘click’, function(e){ blah(e)});

Related Bugs

HTTP Ping Pong

The company has two different sites: desktop and mobile with different domain names. They do user-agent sniffing be with HTTP user-agent header, through JavaScript.

Suggested Fix The issue starts to fix itself by people switching to responsive Web sites. The fix is actually simple. Which ever technique you are using should have a consistent policy in terms of user agent sniffing. Some websites make mistakes when the detection mechanism is not the same on both domains: for example one with HTTP, and one with JS. In HTTP only, the browser is able to identify the infinite loop and will stop after 10 iterations, but when switching from HTTP to JS and then to HTTP, etc the browser is really caught forever.

Related Bugs

Webkit Scrollbar

Websites would like to replace default scrollbars with custom ones. There are known issue of Firefox not hiding the default scrollbar and potential scrollbar lagging introduced with async scrolling (e10s-only).

Related Bugs