Differences to traditional browsers

HTML5, CSS3 and JavaScript are very large standards. We have identified the subset of the standards that are best suited for the creation of game user interfaces. Removing some of the properties/tags allows us to concentrate on the best way to develop UI and provide the maximum performance for those code paths.

For a complete list of what’s supported, refer to the dedicated section for HTML elements, CSS Properties, CSS Selectors, JavaScript DOM Events, SVG Support.

Each new Prysm version expands the supported features and improves compatibility with classic web browsers.

Flexbox layout

Currently, Prysm has full support for the FlexBox layout, which is the most powerful yet simplest layout standard supported by CSS. FlexBox allows creating responsive UIs in a more intuitive way than the classic “Box” HTML layout. For more information on FlexBox, please consult the documentation of the Visual Editor or the following links:

Media Queries

Prysm does not support the media types that are listed in the standard as print, screen etc., because it always displays in the screen media.

// INVALID IN Prysm - screen is implied
/*@media screen and (min-width: 1280px) and (min-height=720px)
        background-color: blue;

// OK!
@media (min-width: 1280px) and (min-height: 720px)
        background-color: blue;

Prysm also supports adding whole CSS files conditionally under the “media” rules. It can be achieved with the following syntax:

<link rel="stylesheet" media="(orientation: portrait) and (min-width: 1280px)" href="sheet.css" />

All the rules from sheet.css will be activated/deactivated according to the expression in the media attribute. Note that @font-face and @keyframes rules are always added. The media only applies to CSS rulesets.

For more information refer to the dedicated section for Media Queries.

Differences with the standard

Prysm deviates from the HTML/CSS standard in some feature implementations. This is done in cases where we feel that a slightly different implementation will give better performance or a more intuitive workflow. For this reason, it is possible that the same page rendered in Prysm may look different when rendered in a web browser.

Layout differences

The most prominent difference compared to a traditional browser is that every element in Prysm has display: flex; and box-sizing: border-box; by default. These are not the default values in the HTML5 standard but are more convenient for UI development.

As Prysm only supports flex layout display: inline and display: block are simulated with flex by changing the flex-direction. There is a difference between the simulated and actual behaviour for these properties so their use is not recommended when attempting to make pages that look the same as a traditional browser. Since the default display for many elements is either block, inline, or inline-block if the defaults aren’t changed there might be some unexpected differences in layout.

The default value for flex-shrink is 0 instead of 1.

<p> and <span> Elements are display: flex and flex-direction: row by default in Cohtml instead of block and inline respectively.

For min-width and min-height the value of auto is always treated as 0.

There will be additional layout(and other) differences if <!DOCTYPE html> is missing as browsers will treat pages without <!DOCTYPE html> as legacy pages and will have legacy behavior for various features.

SVG sizing - Auto-sizing of containers with inline SVG children is not supported, so containers should have concrete sizes.

The <html> element has a width of 100vw and height of 100vh

Another thing that might cause some layout differences is that some less common HTML elements might be missing their default styles e.g. h1-h6, textarea, blockquote, figure etc. However, they can be found here https://html.spec.whatwg.org/multipage/rendering.html and applied to achieve consistency.

FlexBox on all elements makes their layout more intuitive. The border-box sizing means that all width and height CSS properties include the padding and border of the element.

The ResizeObserver API reports the changes made to the observed elements with up to 2 frames delay.

The last difference is linked to the usage of % (percent) on absolute-positioned elements. By standard percentages are resolved against the first positioned parent of an element. In Prysm they are resolved against the direct parent. This is a measure that improves performance and is more intuitive.

Overall to achieve the same visuals in a traditional browser:

  • Use display: flex everywhere.
  • Use box-sizing: border-box.
  • Explicitly set flex-shrink to the desired value.
  • Wrap SVGs in containers with concrete sizes.
  • If a default style of a less common HTML element is different, find the desired styles here https://html.spec.whatwg.org/multipage/rendering.html.

Text differences

Text is one of the areas where Prysm departs from the CSS standard. The HTML standard requires breaking the layout of an in-line box in lines, wrapping the text and allowing for other elements to exist inside. In Prysm we implement single text runs (along with the style) as text boxes laid out with flex-row with wrapping. This has several advantages:

  • Better performance as it doesn’t require a change in the layout tree mid-layout
  • Rendering can be optimized better In practice the difference shouldn’t have a significant impact on the authoring or visual look of the page.

Break (<br>) differences

Currently, <br> tags work only when inside a text run. We have disabled <br> between tags because it leads to bad HTML design and is error-prone. The same result can be achieved much more cleanly with flex items and coefficients.

Window Error event specifics

The HTML standard defines a different number of arguments for the window.onerror = (message, source, lineno, colno, error) =>{}; and window.addEventListener("error", (event) =>{}; handlers. However, the window.onerror handler in Prysm receives a single event object exactly like the window.addEventListener("error", ...); handler.

Custom CSS properties

Color matrix filter

The filter CSS property allows the usage of predefined filters such as blur, contrast and others. Most effects can be achieved with the standard filters, but in case something custom is needed Prysm supports the custom coh-color-matrix filter which accepts a 4x5 color matrix in the form of 20 numbers like so:

.visualStyle {
    filter: coh-color-matrix(0.56, 0, 0, 0, 0.161, 0, 0.56, 0, 0, 0.196, 0, 0, 0.56, 0, 0.302, 0, 0, 0, 1, 0);

The color matrix can be animated with CSS animations. The animation is done by linear interpolation of each matrix component.

The resulting color is computed using the following equation (assuming the input is the RGBA vector):

Directional blur filter

The filter CSS property supports the blur filter, which is limited to applying only omnidirectional blur. Prysm introduces the custom filter coh-axis-blur, which allows blurring per axis. The filter has two parameters: blur values for the X and the Y axes. The directional blur can be animated with CSS animations.

The following CSS code will cause the elements with class visualStyle to be blurred both in the X and Y directions. However, the blur for the Y direction will be stronger.

.visualStyle {
    filter: coh-axis-blur(2px 5px);

Specifying blur only for one axis (direction) is possible and happens by plugging the 0 value for the other axis.

.visualStyle {
    filter: coh-axis-blur(0px 5px);

Here is a comparison between the standard blur filter and the coh-axis-blur one:

Parent nodes

Properties parentNode and parentElement on the nodes are not guaranteed to return the parent when it is not present in the DOM tree and is not referenced from JS.


Event listener callback

The event.target and event.currentTarget properties are only valid in the call stack in which the event handler was called. If you store the event object and use it after it has been fired, its target and currentTarget properties will be null.

Mouse events

The mouseenter, mouseover, mouseleave and mouseout events will not fire unless a mouse move or scroll event is passed to the Cohtml API. The same rule applies to updating the currently hovered element - it will only be updated by mouse move or scroll events firing inside the Cohtml API.

Checking for elements under the mouse pointer and firing the corresponding JS events every frame can result in suboptimal performance. If you need this behavior, you can achieve it by sending mouse events using the cohtml::View::MouseEvent API every frame with the current mouse pointer position:

// Native
    cohtml::MouseEventData mouseData;
    mouseData.X = current_mouse_position_x;
    mouseData.Y = current_mouse_position_y;
    m_View->MouseEvent(mouseData, nullptr, nullptr);

Insert Keyframe rules

The CSSStyleSheet.insertRule() method supports adding keyframe rules, but they won’t be indexed. Prysm will append keyframe rules at the very end of the stylesheet. The index, returned by the method will always be 0. Because deleting a rule requires an index, keyframe rules cannot be deleted at this point.

Backface visibility

The backface-visibility property is evaluated per element. This differentiates from browser implementations where backface-visibility applies to whole subtrees or contexts. If the developer wants to hide entire subtrees they can use backface-visibility: inherit for the subtree of the parent element.