Web Page Layout

This page explains various settings and algorithms that affect web page layout in WebView. The purpose is to provide an understanding of what WebView API methods really do and how are they connected to each other (as their documentation is terse to be said at best), so we can provide proper substitutes for them in our implementation.

WebView APIs

The WebSettings class exposes several options to configure webpage layout.

WebSettings methodDescription
setLayoutAlgorithmSets the underlying layout algorithm:NORMAL means no rendering changes.NARROW_COLUMNS makes all columns no wider than the screen if possible.SINGLE_COLUMN moves all content into one column that is the width of the view (deprecated).The default algorithm is NARROW_COLUMNS.
setLoadWithOverviewModeSets whether the WebView loads pages in overview mode. The default is false.
setTextZoomSets the text zoom of the page in percent. The default is 100% (no zoom).
setUseWideViewPortTells the WebView to use a wide viewport. The default is false.

It seems that the best systematic approach to describe the effects of changing the layout settings, is to express their influence on the following 4 layout parameters that in their own turn change how the page looks on the screen. The layout parameters we will be using are as follows.

Layout widthThe width of the viewport in CSS (aka device-independent) pixels. The notion of a pixel is pretty much overloaded in the mobile world. By CSS pixels, we mean the “pixels” that are used in CSS declarations such as width: 300px or font-size: 14px. These pixels have nothing to do with the actual device screen resolution, and they are not affected by zooming of the content.
Maximum paragraph widthBy default, a paragraph block in HTML tends to occupy the full width of the page. As we will see, layout settings can alter this behavior to make paragraphs fit the screen width.
Initial viewport scaleIn the mobile world, viewport width is always fixed, but user is usually allowed to change the scale of the viewport.
Font scaleAs opposed to the viewport scale, which is applied to the entire page contents, font scale is applied to fonts only.

Hardware Specs

Besides knowing the size of the screen in hardware pixels, for mobile devices one also needs to know the screen density. For example, the screen of Nexus 7 has 800 x 1205 pixels, with density of 1.33. That means, its “natural” width is about 600 x 900 dips. These numbers are used when one specifies “device-width” and “device-height” dimensions for “meta viewport” tag.

The Decision Diagram

Below is the diagram for how WebView sets layout parameters. Depending on the case, some of the settings may or may not affect the result.

Web page layout decision diagram

Elements with a large fixed width can expand the layout width from the normal value (980 or device-width). This can happen when a mobile site includes a huge image for example. We respect the implied width of the page instead of the one stated by the viewport tag; this makes us robust to website administrator mistakes.

Font Scale is only affected by the TextZoom setting.

Configuring WebView like a browser

WebView can approximate Chrome for Android's behavior with:

WebSettings settings = myWebView.getSettings();

System WebView Shell is currently configured with these settings.

Chrome for Android behavior in detail

Chrome for Android works a little differently from WebView. One of the important differences is that Chrome for Android doesn't have the NARROW_COLUMNS layout mode, and instead provides font autosizing. The table below describes how Clank sets the layout parameters depending on the page viewport specification.

No viewport tagLayout Width: 980px
Paragraph Width: Page
Initial Scale: Fit on Screen
Font Scale: AutoSized
viewport content="width=device-width"Layout Width: device-width
Paragraph Width: Page
Initial Scale: 1.0
Font Scale: 1.0
viewport wide contentViewport Width: As specified
Paragraph Width: Page
Initial Scale: ?
Font Scale: AutoSized