Proposed directives
Document-Policy directives such as size-markup, size-images, size-styles, and size-scripts can specify (in kilobytes or kibibytes?) the cumulative sizes of a type of resource; a generic size directive can specify the limit to the size of a page, including all its assets.
Use-cases
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User-agents can know the size limit of a page and its resources just from the response headers of the document. This can inform adaptations for metered connections. For example, the user-agent will know whether to proxy the full page through a webpage-compression service (e.g. Google Web Light), just the images, or none of it at all.
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Authors will be able to specify generic size restrictions on third-party content they do not control. We already have ways for authors to specify image weight (the "max-bpp" directives), but this could generalize it to cumulative weights for resources by category.
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Search engines will be able to more easily evaluate a page's relevance for users who send prefers-reduced-data hints.
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Lazy and conditional loading makes it difficult to estimate the size of a page until the user has scrolled to the bottom. These directives can make ceilings on page-weight explicit.
Further discussion
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How should setting a budget to "0" compare to blocking the resource in the CSP?
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How should user-agents interpret budgets if the user-agent disables images, or disables images exceeding a certain size (common with content-blockers)?
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What other performance budgets should we specify?
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Is there any merit to including an "inf" value, effectively re-setting an existing size directive?
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Should syntax take into account the possibility of future, non-performance types of budgets? If so, should this be split into another header (e.g. "Performance-Policy)? Examples may include a "privacy budget" (part of Google's Privacy Sandbox initiative) in which a UA and a website can specify a preferred budget and the UA can default to the minimum.
Proposed directives
Document-Policy directives such as
size-markup,size-images,size-styles, andsize-scriptscan specify (in kilobytes or kibibytes?) the cumulative sizes of a type of resource; a genericsizedirective can specify the limit to the size of a page, including all its assets.Use-cases
User-agents can know the size limit of a page and its resources just from the response headers of the document. This can inform adaptations for metered connections. For example, the user-agent will know whether to proxy the full page through a webpage-compression service (e.g. Google Web Light), just the images, or none of it at all.
Authors will be able to specify generic size restrictions on third-party content they do not control. We already have ways for authors to specify image weight (the "max-bpp" directives), but this could generalize it to cumulative weights for resources by category.
Search engines will be able to more easily evaluate a page's relevance for users who send
prefers-reduced-datahints.Lazy and conditional loading makes it difficult to estimate the size of a page until the user has scrolled to the bottom. These directives can make ceilings on page-weight explicit.
Further discussion
How should setting a budget to "0" compare to blocking the resource in the CSP?
How should user-agents interpret budgets if the user-agent disables images, or disables images exceeding a certain size (common with content-blockers)?
What other performance budgets should we specify?
Is there any merit to including an "inf" value, effectively re-setting an existing size directive?
Should syntax take into account the possibility of future, non-performance types of budgets? If so, should this be split into another header (e.g. "Performance-Policy)? Examples may include a "privacy budget" (part of Google's Privacy Sandbox initiative) in which a UA and a website can specify a preferred budget and the UA can default to the minimum.