SVG tags with numbers in attributes filtered out by KSES despite being allowed

[johnpgreen]

I have been able to successfully extend $allowedposttags using the wp_kses_allowed_html filter to allow a limited set of SVG tags but have encountered issues when attempting to use the <linearGradient> SVG element. This element is unique among the elements I have added in that it has attributes - specifically x1, x2, y1, and y2 - that contain numbers.

I believe I have narrowed down the culprit to the wp_kses_hair function, specifically that case 0 seems to only allow alpha characters.

[johnpgreen]

FYI - this issue also effects the <line> SVG element.

[ayeshrajans]

Related: #49464.

[rilwis]

I confirm this issue. The linearGradient tag is helpful to use gradient in SVG and is used quite often.

[whyisjake]

Duplicate of #49464.

[whyisjake]

In 48132:

Formatting: Extend wp_kses_hair and wp_kses_hair_parse to allow digits and underscores.

Fixes a lot of issues around parsing XML/HTML attributes.

Fixes #49464.

See #34406, #48608.

Props codeforest, zodiac1978, johnpgreen, dlh, ayeshrajans, johnpgreen, rilwis, travisnorthcutt, miqrogroove, chriscct7, whyisjake.